To be real here, I wouldn't fancy another if I had a girlfriend.
I have had THEM fancy me,,,and then I'd distance my time and me from them,but I also would tell my lady about her,wink,,no secrets..

But ,,hell man, if I'm with a steady,,,I don't LOOK around for romance,,,or attractibilty,,,,,WHY?
Now if we were on the breakup side of together, and we both were aware of that,,,then I MIGHT be giving LOOKS away,,,but still would never CROSS that line of cheating,,to be SURE...

iam, that's the best answer of all.

I love to hear a rugged man such as yourself talk all sensitive and thoughtful like that. Kind of like a man holding a kitten.

If you fancy someone but already have a gf/bf and so do not intend to act on it,would you still be friendly/chatty with the other person or would you keep your distance and rarely speak to them?

I worked with a co-worker back in the day; he was married and I had a b/f at the time..we became close friends, only at work, we NEVER went outside of work not even for a drink...
My b/f who also worked there at the time,didn't like it...so I naturally backed off out of respect for my b/f...
It was a mistake, I never had intentions to put our friendship on a higher level nor did he..we just connected...I felt as though I lost a sense of self since I backed off from communicating with him...
Oh, and the b/f I had, Gone!!...
So to answer the question, It does depend on the individual..

If you love something set it free? Does that apply here?