I think it’s fine to have casual sex with exes for former FWB’s so long as everyone understands the ground rules and is just up for a fling. However, I’m very wary about women trying to fuck me into a relationship. It seems to happen quite a lot with younger women where that take sex as a sign of some kind of implicit commitment.
If you know a woman wants commitment or is into you romantically and you still have sex with her casually (from your perspective) I think that’s a bit of a dick move. It’s kind of taking advantage of her emotions. Even though she should perhaps be smarter and more protective of her own feelings/self, it’s still selfish on the guy’s part if he leads her on to thinking there might be something more than the purely physical.
The problem is, girls can send very mixed signals about whether their feelings are physical or emotional and what it is they really want. So you can be damned if you do and damed if you don’t.