If she goes out with other male friends regularly but only this
particular chap is making you feel uneasy, then trust your gut. You
have no real confirmation whether there is something fishy going on
between them but dude, this line alone is bad enough - when your
woman is ok with a cooling off period without putting up a fight or
wondering why, her interest in you is waning. The real problem
isn't the guy, because even if you manage to bash this one with a
stick, there will come another, and another.
Fretting over her 'guy friends' is not going to help. And it is
true that men don't keep pretty girls around for the sake of
platonic friendship (unless they are blatantly gay), but by making
her promise never to see him again you are showing her your
insecurities and insecurity in a man is a bit of a turn off.
Besides, if she wants to see him, there is no way you can stop her
- it is impossible to keep any woman on a leash.
The biggest mistake you can possibly make dude, is making her
choose between you and the boys. Why don't you demonstrate some
confidence by telling her you are glad she is hanging out with
these boys and if she wants to go out with a hundred others, she
should be your guest and go ahead, play with them and have a good
time?
Since she is perfectly fine with not seeing you for a month and
then take some time off, don't call her, don't send her messages,
make zero contact and find your life outside of this heavy duty
relationship.
Just my two cents worth.