If you never played the other paths where Lena has sex with other people, then you'd never know the difference. You are looking at it from the point of view of the player, who has previously played Lena as a slut. And yeah unsurprisingly she gets a lot more content, and if you then miss that out in a future playthrough where she remains faithful to Ian, it might seem like a lot less of a game. But if you only play Ian and Lena having sex with each other, and nobody else. Then you've got no additional content to draw a comparison from. And what you claim above, is the opposite of what actually occurs.
Lena can only be made to be a slut, if the player ultimately has no choice, but to make her into one. That is obviously not the case, because you can totally play her as a faithful partner, without ever feeling that your missing out on anything. Because the only way to know that, is by making different choices.
