Is FWB gonna end up with my heart broken?

(Not real names)
Mark: guy I’m getting more serious with
Frank: FWB

So basically I’ve been seeing Mark about a month. We haven’t established that we’re serious yet but it is getting serious. I have a feeling he’s gonna ask me to make it official soon. I do really really like him. He’s a beautiful person, but I’m not sure if I really want a serious relationship. Well, it’s not that I don’t want one, it’s that I don’t think I should be in a serious relationship at the moment where my mental state is at. Frank and I have been friends for a couple of months. The other day we hooked up. There’s no denying he’s attractive. I can see myself eventually being with him to be honest, but neither of us are interested a relationship right now. Mark is such a beautiful soul, and he makes me happy. But something about Frank makes me feel so free. I guess I haven’t had too much time to be single in the past 3 years or so, so I feel like maybe that’s why. I’m afraid if I tell Mark that I’m not interested in a relationship he won’t stick around. But I’m afraid I’ll catch feelings for Frank and end up with my heart broken. Any advice?

TL;DR: I really like one guy, but I’m not ready for a serious relationship. Another guy who I hooked up with once makes me feel so free. He’s also not looking for a serious relationship. Will I end up heartbroken?