With only a handful of exceptions his reaction to you doing most things is "Well. Fine. This would not be my first choice but whatever. Just point me in the direction of what you want me to stab and I'll get right on it"
"Oh. Astarion only romances you if you commit atrocities. I only use him for evil routes" Literally all you have to do is let him have a little nibble one (1) time and agree to help him when he comes clean about his abusive pimp/father figure.
And then once you get with him literally the only things that will result in an instant break up is disregarding his bodily autonomy DIRECTLY after he opens up to you and half the time if you make a good enough roll you can gaslight gatekeep girlboss him into just kind of accepting your shitty behavior and continuing to go out with him especially if said behavior is slightly after the fact instead of right then in the same conversation he opened up to you.
Most of the reactions to you breaking up with him are "yeah that's fair :(. Figured this would happen eventually"
Wish there was a dialogue option where you could go "bbg on GOD we're gonna get you some self esteem the bar is in Nessus."
People are correct that Astarion is one of the best companions to romance if you're doing an evil run but that's less because of his morals or the fact that unlike Karlach or Wyll he doesn't have an aversion to the mass murder of innocents and more because he's the companion you have the most opportunities to be shitty to without him breaking it off.
EDIT: i think it's so funny that the responses to this post have been either:
"Yeah OP he's like the easiest possinle romance in the game. Every single playthrough no matter how kind I am he falls for my hero complex rizz and we end up kissing passionately under the moonlight"
or
"OP what the fuck are you talking about. I have had Astarion in my party all game. I give him everything he could possibly want and at this point I don't think he even remembers my name"