i have been with my gf for 5 years now and its awesome she knows i have done stuff with guys before her now i wanna get back to having sum fun with mates but dont know what to say to her or how to say it
'Hey hunny, I bought you a nice new pair of shoes Oh, by the way, totally going to sleep with men again, that cool?' Nah, just tell her. If she knows you did it before shouldn't be worried about what she will think of you, should be more worried about how she may feel about you going off with other people, regardless of gender.
I would never tell someone I'm dating something like that. I would ask them how they feel about it... but there's no way I would tell someone I love "hey, I'm gonna go bang some other people"
I'd bring it up gradually. It sounds stupid, but bringing it up jokingly at first does give you a chance to gauge her reaction. If she's against bringing other people into the relationship regardless of gender then you might be out of luck