I think this is more of a submission thing (given that the other way to enable it is for Ian to be dominant in their relationship). Alison being a little older generally seems to treat Ian as someone who should follow her lead, but if he leaves her wanting for long enough or figuratively fucks her brains out, she comes to accept that Ian is the person who wears pants in this relationship and she is his fucktoy to do with as he pleases.I really gotta believe it's baby-trapping by this point. If she can't have him to herself voluntarily, well, she'll make sure to nab him one way or another.
Like, on the blonde path Ian can outright call her a bimbo and she'll just take it. The only point where she tries to make an objection (but still folds) is anal sex, and that's only because it's genuinely too painful for her, apparently.