Let me state from the off that I realise why sexual orientation is important to players but I am purely thinking about the game world of Thedas. It is my understanding that there is no prejudice against homosexual relationships in Thedas. In Masked Empire Celene is being criticised by her enemies not because she has a same sex lover but because that lover is an elf. Even Gaspard's comment about her being lesbian accounting for why she refused his suit is surely more to assuage his ego since he knows full well that marriages among the nobility are done for political or economic reasons, not simply sexual attraction. We also know that Celene was perfectly prepared to marry Cailan and presumably at some stage that marriage would have to have been consummated for it to be valid. Besides the fact that Eamon was suggesting dumping Anora for not providing an heir so at some point Cailan would expect to bed Celene.
However, I was trying to think of situations where the sexual orientation of a character might cause problems. The first that came to mind was in the City elf community, which could be relevant to Sera. In DAO we are under pressure to marry for the sake of the community and these are arranged marriages of the opposite sex, clearly with the intent of producing children. You can object all you want about the partner that has been selected for you but you seem to have no alternative but to comply or be a social outcast.
In Thedas, homosexuality is viewed differently depending on culture, however no one culture in Thedas as we know of is really that hostile towards homosexuality. interracial love is really the hobby horse that everyone has jumped onto to lynch people for, not some man who finds out that he prefers eating a bag of apples rather than licking vaginas and vice versa. As pointed out in previous posts, in Ferelden, homosexual relationships are seen a wierd and odd, but are not looked down upon on a personal level. Orlesians don't really have any strange thoughts about homosexuals. It's not uncommon for Antivan Crows to have multiple relationships upon members of both sexes. The Chantry has no real moral viewpoint on homosexuality nor do the Qunari. Among nobility, there may be some parental and/or social pressure to carry on a heterosexual relationship to continue the noble family's bloodline, but this is more of a pragmatic viewpoint rather than a moral one. Dwarves and elves are dying races, so they most likely of all people will most definitely want as many heterosexual relationships among their own species as possible to continue their existances. Although both don't seem to have any problem with members of their species carrying out same-sex relationships among each other, they may quite possibly frown upon them, though they probably won't lynch you for such a thing, it is probably a somewhat social pressure among dalish elves, city elves, and dwarves to continue producing children in the numbers since dwarven fertility rates are pretty low and elves continue to be a dying race inching closer and closer to extinction by the day, however that same pressure would apply to those who don't wish to ever be married, not just to homosexuals.
I also thought that the Dalish, rather like the city elves, might put a degree of pressure on their members to pair up with the opposite sex because of the need to keep their numbers up but they don't seem to have arranged marriages in the same way the city elves do. That elf girl (forget her name) was able to refuse her admirer's suit because he had not yet proved himself, so clearly being able to provide for your partner and any children is the priority in Dalish culture. Which made me consider another thing, there is no polygamy in Thedas. In a culture like the Dalish, where for very understandable reasons you need to be able to prove yourself as a hunter to be acceptable as a suitor, why do not the more able hunters have more than one wife?
This would make some sense to some level, if there is a need to keep numbers up among Dalish clans then why not just let the bi and straight males mate with multiple women so long as the women continue squeezing out the crib lizards in masses? It makes perfect sense, but from a biological and maybe psychological stand point, maybe not so much. I'm not a chick, so I can't say this from a personal level, but I've been told this time and time again is that what a woman wants to make is love, rather than sex. In simpler words, that means that they would much rather be having sex organs inserted into them because you love them and she loves you. They both involve the same thing, which is sexual intercourse, but the woman in question will almost absolutely want the intercourse to motivated by love and not lust. That's not to say that their aren't women out there who enjoy having sex organs being inserted into their bodies or having money shots on their faces merely for the fun of it. A quick look at any porn actress can make this abundantly clear. However, keep in mind that this one or possibly more crotch dropling(s) coming out of this one woman will require her care and housing, which can be a lot of work for one just one woman to do, so if she's going to have to know that daddy is possibly going to have another batch of kids on the other end of the dales to help bring up, then that might have the woman in question feeling a bit cheated. From a biological stand point however, monogamy reduces a chance of spreading STD's out rampantly, if one infected male goes out having sex with 50 other women, and let's say 30 of the women end up with the STD, that means that there will be 30 more STD infected kids possibly coming and contracting their own STD's onto partners, so you can see how this has a bit of a snowballing effect so you can see how this can sort of be at least kept to a minimum by limiting people to one partner.
A second example might be the Grey Wardens. They clearly have no problem with relationships outside of the Wardens, since for the most part you are meant to leave these behind. However, how would they view relationships within their ranks. Would they frown on any relationship because concern for your lover might put a mission at risk? Would they feel, like the Spartans, that a m/m relationship would strengthen the bonds of brotherhood? Or would they regard it in some way as a sign of weakness? I still can't get over the fact that they made Anders get rid of his cat, so this idea isn't as daft as it sounds. Incidentally I seem to recall that women are not nearly as common within their ranks as men.
Grey Warden women are already infertile to begin with, it comes with them becoming a Grey Warden, so there would ultimately be no ultimate setback to female Wardens having relationships with other Wardens just so long as they don't interfere with their duty as a Grey Warden. The same can be said among male members of the group. Even among the male members of the group, having sex is not advisable. Wardens like the dwarves are infected with the taint of the darkspawn, so concieving a child is made very difficult and if both of them are Wardens, the chances are next to nothing. This also comes into the fact even if the Grey Wardens manage not to get killed battle, they most likely will not live past middle age, which makes them very poor candidates for long-term parenthood. Grey Wardens may be able to enjoy a certain reputation for sexual prowess though. Duncan had a brief affair with a Circle mage hinted in Dragon Age: The Calling. Furthermore, with that said, I think it would make sense to know that, Wardens don't really care about other Wardens having relationships with others, let alone who they are having a relationship with, just that it doesn't affect their duty as a Grey Warden.
Alternatively, why does it have to be the male who proves themselves to the female and not the other way round?
I've pondered and thought about this myself many times to myself and could never quite understand why a woman will go about trying to get a man's attention rather than just approaching the man herself and trying to prove herself to him. This certainly doesn't make any sense to know this and doesn't seem fair either, but whether it's a stick up the ass called tradition or if it's just some way women are wired, I really cant say. This however can vary from culture to culture, there may have or be some culture out where the female proves herself to the male, but if such a culture did/does exist, I don't know of it. I do know that in Ancient Greek times, polygamy was totally normal and that one Greek male could "possess" multiple females and such attitudes would spread to the Romans until Tiberius dropped the axe on such a thing which would become a philosophy that stands today, so you could say that women were ultimately "proving themselves" as legitimate bond material to the males, but such a thing can be debated. However, such a thing could be argued that their is some sort of "proving yourself" value among women to men. Have you ever noticed how women can become very catty and mean to each other when they both like the same guy? Have you ever noticed when a woman is interested in a man and then when another woman shows interest in the man and the woman precieves the other woman in question as more attractive than her, she then ultimately goes to hate the living hell out of the other woman out of pure jealousy? This may be somewhat of a signal, that there may indeed be a little bit of a sense of a woman proving herself to a man.