Maya doesn't really stop anything because, if you play it this way, she not only willingly lets the MC finger her and suck on her tits but she also fully gives herself over to her feelings for the MC at the end of ep 3 and was willing to have sex with him but the baseball bat was too much for her at that time.
Her Dad clearly knows about her and Josy and the "ultimatum" Maya mentions was likely something along the lines of him trying to make her give up the relationship. I can't remember if this was explictly stated or if I'm just inferring based on certain things that have been said, but I think she comes from a very religious household, so this would explain, although not excuse, her Dad's actions.
Maya also wasn't bi, or at least didn't think she was, until she met the MC. She's still not sure if she is or not at the end of ep 5 and perhaps that's what she wants out of this "open relationship" proposal; to see if she feels anything for other guys or perhaps she just wants to see how she feels if the MC is with other girls. Now that she has Josy back, she might think that what she felt for the MC was a fleeting thing brought on by her loneliness and so by essentially letting him pursue others, it's a test of her own feelings for him; will it bother her to see him with others, and if so, is it just petty jealousy or does it run deeper than that?
In the end, I hope there will be a happy ending for these 2 with the MC. It's an unconventional relationship, but not unbelievable as relationships like that do exist in the real worls, and after AL set up a throuple and then destroyed it, I would hope that we will get a happy ending with these 2 to make up for that.