It's not a mistake, I'll tell you more, it's not even the beginning of their romance. At the beginning in DOS2, you can also sleep with Fane "for the sake of science" and this is not the beginning of his romance. You sleep with Lae and Astarion more for pleasure than anything else. You can literally choose the "want to have fun" option in the Astarion cutscene. Take it easy, you can always say no.
Also on a neutral level, I know 100% that Lae doesn't suggest you sleep with her, she says that you "often turn your back on me" or something like that. in general, a neutral level is not enough. Astarion can also reject you with the words " you're not my type." So you either have an approval bug, or your approval is higher than neutral.
