If a guy says "like if you would be my #wcw & he's in a relationship, is that a bad thing ?

Yes, that's a very bad thing. He's basically asking to mess around on the side. This means he's A) a cheater B) a user and C) has zero respect for you as a human being. But if you only want to mess around with him, that's fine. Just use protection if you go that far, ok?