It all depends really, but I think a guy would cheat on a girl faster, so I'd say girls are more loyal
It does depend on the boy or girl. But I think humans ARE instinctively monogomous. I hear people say that we aren't,but in the thousands of years we've been around, we are all still searching for the one.
I think that may be from both side and it's totally depends on the honest and believe of the partners with each other. No one can predict who would be more loyal.