The period between being turned and the ending scene is one of profound unease and growing suspicion. Yes, even if you never pick these dialogue choices, the game decides you're just not letting these feelings show. Your feelings are decided for you. That usually happens if a lead writer has very strong opinions of what's going on, and they're not well translated.
I also got the impression that how the writing envisions the players feelings towards Astarion, opposed to what the players who romance him actually do see in him and the relationship can differ quite considerably. When I look at the one consistent characterisation of the PC in the romance (and that panel that got linked a few months ago seems to support this) they are mostly interested in Astarion's looks and in sex with him. They might also have come to care for him but mostly in a way that implies keeping him save on the spawn route and leeching of his powers in the ascended route. Seeing him as a truly valued partner (what the graveyard dialogue states him to be for you) is something you cannot really express in either version of the romance, though I got the impression that this is what many people actually do see in him.
