Sebastian is not a virgin. He has his wild years before becoming a "chaste" person. I don't think Kaidan is a virgin either, he never comes across that way. It's just the way he carries himself, and talking, and how he is in a relationship. He did go on dates with a girl people set him up with, he never said it's the first time. I'm sure he had sex before. DAO Cullen is definitely a virgin. I'm not sure about DAI though. Basically, the only people we can say for sure is Alistair, Cullen is a very high chance.
Danse is a jerk, but it's not because he's an unpleasant person, but it's just more about the people you hang out with are all people who he disapproved of. The people under him are sided with him and respected him. Even after he turned out to be a synth, many of them still believe in him in secrets. I don't think going by what he said to the other companions is a good indication of who he is if he conversed with other humans. He has no problem with Cait and Preston. He was more concerned about Piper because she has no combat training and she tends to poke her nose around a lot, she herself said she almost got killed twice because of it. He's a lone wolf, but it's just more like because he's an introvert and tend to focus too much on work. He's like a foil of Alistair, I guess, since Alistair likes to slack off, doesn't like to take charge, childish, straight. Danse is serious, focus on his work, take charge as paladin and have to give out hard decisions and stick by it, mature, he's bi (I headcanon him as gay since he's the only person that's the most likely to be in that spot if we were to give LI set sexuality
).
Actually to me, Danse is an unpleasant person and you are making excuses for his behavior. "It's just more about the people you hang out with are all people he disapproved of". Really? So it's everyone else's fault that he looks down at them with disdain or is downright rude when he knows nothing about them. And is he really concerned about Piper? When you switch from Piper to Danse he has a harsh cold tone and is frowning. He tends to do this around people he just doesn't like. In the reverse he doesn't show concern - he is looking down on her and is insulting her. And she is rightfully offended and responds accordingly. This isn't the MacCready/Deacon "stay safe" comment that seems more sincere. He's just being petty that he isn't staying with you and he feels superior to her.
As for Kaiden, he went out for drinks with one person which he said he did only because he was set up and it went nowhere. By ME3 if not in a romance, he says he never made time for one. The only other girl he mentions - it doesn't sound like they were an actual item. Just someone he was into who he scared off in biotic school. He doesn't have that God-awful "did I do it right" conversation Alistair had, but he doesn't come across as being experienced either. He just at the very least comes across as hot celibate guy until you come along.
And Sebastian I see as more of a rogue and not a KISA. He was a playboy. Maybe you can see his personality has shifted into more of a KISA but so has his sexuality. He is now chaste and won't sleep with even the PC. Most feedback on him was very negative and I'm sure this is not really who anyone thinks of or wants the next KISA modeled after.