Ah, yes I read about that option. I wouldn't go for it, but while I can't speak for Alistair, there's nothing wrong with exploring your sexuality. I would just interpret that as him being more open-minded. And while I don't want to start an argument, I wouldn't go around throwing derogatory slurs at characters like that. Isabella's sexuality [or anyone's for that matter] has no bearing on whether she's a good person/character or not.
I like the way how people try to transform what I say or always misinterpret it. 
I absolutly don't care about the sexuality of Isabela, she does what she wants, I don't like her because she is dishonest, is a thief, manipulates people (with her charms or not), doesn't like it when you try to help people in need, etc. I don't like the way she acts. (edit: I'm not an english native speaker, is the word "slutty" always linked to sex or can it mean "a woman who is acting in a mean way"? because in french we use the word "salope" not only for sex but also for manipulative/dishonest women, and slutty was the translation for it. Maybe in english the word doesn't have these subtilities)
See, I really love Zevran, so my liking of a character has nothing to do with sexuality, don't make me say what I didn't say, please.
What I didn't like about the threesome is not the fact that it is a threesome, it's the fact that Alistair was really totaly against it (and it's not because he's narrow-minded, some people are jut not into that, they are free to like this or not) and suddenly, you say one little thing to him and BAM, everything changes, he suddenly wants to do it, and that's not credible at all (that and all the other consequences of the hardening, it makes you think that someone changes totally if you just say one word to him. His personality, everything.) Hardening him should have been with different steps along the game, not just one dialogue option and Tadaaa, Alistair totally changes!
And someone said that hardening him made him say "no" to the warden when she wanted to spare Loghain. Well, hardening has nothing to do with it because he also says "no" to the warden when he is not hardened and he leaves. The only difference is that he doesn't become king, but he still says no. So even unhardened he already had the temper to say "no, I don't like the fact that you want to spare his life, so I leave you warden".
And I didn't like the fact that he would consider cheating on the warden in DA2 if Isabela proposes something, yes. I like romantic romances and cheating is not romantic to me, sorry people. (he's already cheating on Anora but it was an arranged marriage and they don't love each other so that's a bit different).