Um, if you flirt with someone, and they turn you down for whatever reason (be it because they're committed, not interested in your gender, not interested in you...), it sucks. Especially if you really are interested in them, it can sort of feel like a downer, and probably will make you a bit upset. It makes perfect sense for Anders to be upset if you turn him down, almost especially because he's so emotional.
You know, the weird thing is, I've been going through the sound dumps. And honestly? Sera does mention finding woman attractive more often then Dorian does. Dorian, it comes up in three places: in his quest with his dad, which is not about him being gay (it's about him refusing to enter an arranged marriage with a woman he doesn't like and breed for the sake of a bloodline he doesn't like and his hurt over his father's hypocrisy and anger about blood magic), in his romance with Iron Bull, and in his romance with the male Inquisitor (oh, and if you choose to ask him if he slept with Felix- but you can ask Leianna if she slept with Justina, too, which is just as ridiculous). He doesn't spend time hitting on guys (notice that in both his romance with the inquisitor and Iron Bull, he is the one persued- he doesn't make any first moves, and in paticular with Iron Bull, takes a while to take him seriously). He doesn't spend time talking about guys. Most of what he talks about is Tevinter and magic. I mean, listen to the banters: if he's not in a relationship, his sexualty doesn't seem to come up once as far as I can tell. The time spent on his character's sexualty is about as much spent on any character's sexualty. If he WAS straight- don't you think he would have still said screw that to the arranged marriage and breeding with a woman he hates and ran? His quest would have been the exact same, and his father still likely would have tried to use blood magic to make him do his duties. It's not a quest about being gay, it's a quest about where your freedoms and rights stand on the scale against family duty vs BLOOOD MAAAGIIIIC.
Sera on the other hand can hit on your Qunari female pretty squarely (and if you're not a Qunari female, mentions she'd like to meet one if you ask about Iron Bull because 'woof'), remarks on Lei, Cass and Celene's attractiveness, seems to sort of be inclined towards Dagna as well (if the giving a nickname and plotting awesomeness means anything) and talks about seducing noblemen only to tell them she likes their wife better (...which in her mind, is imitating Josie, in a banter that amuses me terribly).