If Lena was such a slut, she would have started posting naughty content on Stalkfap, as soon as Ivy told her about it. She wouldn't have waited months to actually build up the nerve to finally do it. She only becomes slutty, because the player corrupts her by degrees, over an extended period. It's never a forgone conclusion.
She doesn't really want to have anything to do with Axel. The player has to make her want him again. She never really fancies Robert at all. Once again the player has to make her indulge him. Mike is there, as the alternative to Ian, on routes where they never get together or split up. Yeah, she can end up having a slutty, cheating relationship with him as well. But to claim that's his only purpose, is just false. He's also a possible future love interest for Lena. With Jeremy, Lena has to obsess over him for quite sometime and only if you choose for her to love big dicks. You have to have her jump through a number of hoops to get anywhere near Jeremy. If she was a slut, why would she ever hesitate?
Marcel requires her to fantasise about big cocks, Seymour pretty much blackmails her into indulging him, with Jack she needs to flirt outrageously with him and she can only get Mark if Ian is dating Holly and not her. It's impossible for Lena to have sex with any guys but Ian and Robert, without the player consciously deciding to make Lena into a slut. So the notion that it's some kind of inevitabilty, is just plain untrue.