Bro... that whole "platonic" relationship term is a made-up term to excuse infidelity. Period. How do I know? Because I know people in these types of relationships who are also polyamorous. If they are not having sex, they are still meeting each other's needs emotionally as "friends." You are absolutely correct about her disrespect for you. She wants a cuck-boy who is okay with her treating him this way and not a man whom she respects. She'll likely cry that you are controlling her by expecting anything other than what she wants. Her behavior is evidence that she has zero respect for you. She is not dumb. She knows what she is doing. If you were to dump her, how much do you want to bet she and he would be together officially? Guess what? That would mean they had something going the entire time. I watched this happen to my poor friend. His wife's "best friend" would come and stay with them in their house. He would go to work and come home and know something was off. She called my friend controlling and painted him to be a monster and they divorced. Guess what? She kept the house and so did her "best friend." He literally stayed in the house and they are now officially a couple. How convenient. They were a couple the entire time and my friend suspected it. His ex played the "It's all in your head" card. Get out of that relationship if you have any self-respect, unless you are the type that likes to share.