What happens when you get into bed with someone who has, say, stretch marks that aren't visible when they're clothed? Is that a turn off? Or do you not care by that point?

The latter... by the time I'm getting into bed with someone, I know them, and are presumably attracted by their personal qualities... initial view-at-a-distance filters are secondary effects by then.

Besides, I've never been to bed with anyone that *didn't* have hidden stretch marks! I expect them... I have yet to date anyone smaller than a size 18... not because of a BBW preference, it has just happened that way.