Boyfriend's cat sleeps on my legs at night. Is this a sign of affection?

With cats, it's really hard to tell! It's probably a very simple reason, maybe your boyfriend just moves a lot in his sleep so she can't sleep on his legs. Whatever reason it might be, the cat is probably pretty relaxed around you if she trusts you enough to sleep on you, and that is quite an achievement on it's own! If you're really curious about the affection or dominance thing, just try to notice how she acts around you in other situations.