My main theory as to why she would fuck Denims is based on 3 reasons:
1) Wanting to worm herself further into Hasan's circle, either to be with him or, should that fail, be able to benefit from his fanbase and possibly monkey-branch on one of the richer people.
2) She wants to experience sex with a woman once or twice while she is still "hot".
3) She had needs but will choose to avoid sleeping with a man in order to avoid getting pregnant. Money is tight to have an abortion and hide it from the world. And Ian might figure out the baby is not his which would increase the ridicule both get online. Maybe she does not want to risk Ian leaving or killing himself before she can monkey-branch.