However there could be a romance where a straight guy could realize that he actually is a gay because of the gay inquisitor (or female inquisitor). But if there would be these kinds of plots all the time then I think that the romances would become repetitive.
Yeah, I actually wouldn't mind a "coming to terms with their sexuality" LI. I've seen other people express that they'd rather not see it because it can go really wrong really quickly, but I have faith that Bioware could do it right. Although, it's not terribly realistic to do a "straight guy realizes he's actually gay", but it could be more of a closeted guy comes to terms with his sexuality. Maybe a guy who, with the initial flirt, is like, "Uh, no. I'm not interested in guys like that....?" and then over the course of the romance he starts to question himself and the PC and reveals that he's always thought about it, blah, blah, blah. It might be interesting. The only LI that was kind of like that, in my memory, is Sky from Jade Empire. It might be interesting to revisit it with a different character.
Do you have solid arguments for supporting bisexual approach outside your personal preference?
Well, no, to be honest. I'm actually more of a 2/2/2 split guy. I don't mind having one guy who's not available to me. I have no problem with the way that ME 3 approached it with the glaring exception of the terribly unequal distribution of LI's across gender and sexuality. I don't think we'd have that problem in DA: I, though, since we are all starting from a clean slate as it's a new PC. The other concern that I have with it is that they tend to put the "main" romance as the straight one. If they could subvert that practice by making Cassandra bisexual (or even lesbian), that would be great. I know that some people are really against the idea, but it's an "agree to disagree" thing.
Regarding the "all bisexual" approach, the best argument for it is the fact that it makes the romance mechanic much easier from a gameplay standpoint. There are just four LI's and there are no flags, so everyone can access everything with just a few changes in dialogue here and there. It's not my preferred approach, but it's definitely fine and, for me, the big positive side effect is that it increases the number of bisexual individuals that we encounter, which is always a good thing.