My best friends are guys but I'd never go with any of them, I'm perfectly happy with my boyfriend. My best best friend (T) is male and we are very close, I see him like I see my brothers and we are always together. My boyfriend is fine with this as he trusts us and T is probably his closest friend so we do loads as the 3 of us.
Trust what your girlfriend says, even if they fancy her it doesn't mean she likes them back. If her friends are anything like mine, you don't want to get on the wrong side of them, mine are really protective (v.annoying sometimes!) and would kill my boyfriend if he ever hurt me.