Completely agree with OP. One of the PLAYER dialogue choices in this conversation should be supportive and positive. Astarions dialogue doesn't need to change at all. I'm kinda fed up with being moralised at. I choose him because in my role play I love him. I didn't know being a vampire like him was even an option when I ascended him (we never spoke about it before) at the time this conversation lands he hasn't even offered it. I spent most of Act 2 and whatever portion of Act 3 I'm now at being celibate, so he could get over his trauma so wanting his body wasn't a main motivation for anything. He can call me pet, or any other endearment, whenever he wants and I like it. So why on toril am I now being all hurtful and confrontational when he isn't trying to pick a fight and I'm certainly not? I loathe this dialogue.
