I also used to think that Diane was a possible LI since she was always flirting with the MC, but now that seems quite unlikely, i think that it was rushed to have sex with her now, and that scene has 0 passion, (This scene and "arrangement" would suit Lisa way better than with Diane) , but i think that the dev did that on purpose, i know that Lovers and fuckbuddys are not the same thing but, if you agree to become Adriana lover she says "no attachments boy" ,and yet every time with her she is full of passion towards the mc even more than Sandra who is the actual GF, and that makes you think if she will want something deep in the future, and Diane scrap that out imediatly so yeah i think the dev did that on purpose to let it clear that Diane is not a LI.