I have lots of female friends and they in turn have lots of guy friends. Like I've said, it is perfectly possible for a guy to be friends with a girl; owing mainly to mismatched SMV, personality, intellect, or usually a combination thereof.
There are types of 'guy friends' that are red flags however, for different reasons. It's up to the discerning gent to reconise these and act accordingly to the situation:
-The Ex BF. In short, being friends with an ex is BS, nine times out of ten. They likely still hook up every so often, or at least might.
-The guy with equal / higher SMV (to you). Pretty obvious, he's an(other) option for her.
-Obvious Orbiter. Obvious Orbiter is obvious because he's obviously wants to get laid, but isn't, instead is being employed for some benefit, weather either party realises or not; it is not a good reflection of either.