I haven't been on the forum for a while, but I thought I'd throw in my $0.02.
We all need affection in our lives. It's one of those things that keeps us smiling. Affection can be given (and received) by anyone. What you need to realise, is that it is easy to misinterpret affection by others, as an attempt to 'steal her away', or as 'cheating'. I don't know the guy in question, but there may be no such intention, and it could be entirely innocent.
I have this situation myself, with a friend who lives up the road - she knows full well that I have a girlfriend, but we regularly cuddle up on the couch together anyway, and you know what? NOBODY WORRIES ABOUT IT, because it's nothing more than us showing that we care about each other.
So basically, you might be worrying yourself over nothing.