Maybe it didn't happen in your playthrough, but it can happen in others. Which means within in this game, Lena, Ian and Holly all have the potential to be more promiscuous than Ivy. It may very well be, that outside the game Ivy is fucking around, left , right and centre, but so far the evidence is all circumstantial. Whereas, with these three we can actually see it.
For instance, it's possible for Ian to have sex with Gillian, Lena, Cindy, Alison, Emma, Cherry, Minerva, Louise and Jessica in a single playthrough. Whereas Lena, can have sex with Ian, Robert, Mike, Axel or Jack, Seymour, Jeremy, Holly or Mark,Louise and Ivy, plus another guy she meets at Blazers, and give Marcel a BJ. And then Holly can have sex with Mark, Robert, Lena and Ivy, plus gives BJ's to about 5 or 6 guys in a gloryhole. Ivy on the other hand has sex with Lena and Holly, Mike out of picture, and gives Jeremy, plus one guy in the gloryhole a BJ.