So, when I played through the first time, from Lena's comment and the artwork, I thought Holly was cock-drunk. I was remembering also that Holly's stated fantasies earlier in the game had involved a stranger fucking her from behind, and though that didn't happen here, these were all strangers to her and she was probably getting off on that.
Second time through, it occurred to me maybe Ivy had given her drugs. Probably not against Holly's will, but in this route Holly has been well accustomed to submitting to Ivy's desires whether she really wants to or not. It's also completely possible, as has been said, that Holly was interested in trying the drug. But at the moment it's all very unclear. (It also lends support to BlandChili's idea that it would have been better for Lena to have been present with Ivy and Holly, so both she as a character and we as players know better what happened.)
I do hope Eva steps in and makes things more clear through some dialogue, if not in the Beta or Final, then in the next update as Lena and Ivy are talking on the way to Billy's place, or whenever Lena and Holly see each other next.
This may be interpreted as defending Ivy, and I'll take that risk, but from a story standpoint, I have to admire Eva's ideas of progress in Holly's corruption. A glory hole experience makes logical sense (oof, do I really want to be saying that?) as a next step for Holly. It's multiple partners, which is new for her, but casual and she doesn't have to see them again. It's multiple partners but no penetrative sex at all. It's "secret," as in there's nothing connecting Holly publicly to the act, as opposed to something like doing a stripping session at Blazer, which is honestly what I had guessed would be the next thing Ivy would want Holly to do.
