I have that opportunity right now. But I do feel like she's only interested in my looks (or mainly interested in my looks). I know for a fact that I could probably do anything with her if I wanted to. But I don't, because even though she's really cute, personality-wise and looks-wise, I only want to do something with her if it's actually gonna mean something.
A casual relationship? Please define this more explicitly. If you're talking about a casual relationship with some physical qualities to it, then that's a different question entirely. If that is what you mean, though, I'd have to say be careful in that situation, as people do often get attached and whatnot. Of course, I'd have to deny it, as I'm already in a relationship. However, if I weren't, and being a guy (guys love sex, never forget that), I'd probably be inclined to accept. Although my last relationship started off that way and I have to say it was more detrimental to it than anything, as we've now had our fair share of problems, and I think that's to blame for it.
In looks only, meaning they just want a FWB thing? No way. But if they approach you based on your looks, and genuinely seem interested in pursuing a relationship with you rather than just sex, most definitely.