tldr- Been FWB for 4-5 months now. Girl is starting to want LTR but she still talks to an ex and they hook up every so often. Doesn't matter to me in a FWB but that's the only thing making me not want to move on to LTR. Not sure how to address it.

We've been FWB for 4-5 months and a LTR is looking like the next step. She hasn't pushed it too much but she's starting to drop hints more often. Throughout the time we've been seeing each other, she's been in pretty constant contact with a guy she used to date, hooking up at least once since we've been seeing each other (may be more, I only know for sure about one that happened about a month ago, I don't see it being more than 2-3 times though)

She doesn't know that I know about they hooked up last month. I assume she knows that I know they stay in contact because she doesn't hide it from me or anything. I've never brought it up because we weren't exclusive and it really didn't matter to me. But now that a LTR is looking like an option I'm not sure how to handle the situation. Her being in contact with the ex is the only reason I haven't gone into a LTR with her. I could tell her that I want a LTR but only if she cuts off all contact with him, but that just seems super BP to me and basically gives her all the power in the LTR at the very beginning. If I just continue with the FWB she's going to start hinting at a LTR more and more and, without her knowing the reason why I haven't initiated it, she most likely will just continue to stay in contact with him. Just prolonging the process until I eventually have to tell her why I haven't gone into a LTR with her anyway.

FWIW, I truly think she'd stop talking to him if we went into a LTR. It's just a risky move to jump into a LTR when it's entirely possible that they keep in contact since I haven't had any issue with it until we get into LTR territory.