Am I really cheating on my girlfriend?

So I've been with this girl for 8 months now and it's starting to get serious, I think that's the right way of saying it. Basically we've gotten really close and things are going good. We're also both 22 and in our final year of uni, for some perspective I guess. But I've also "been with" my best friend for much longer. We've been friends for like 12 years and in the last 4 or 5 we've been a lot more than friends, if you get me. We're not officially going out or anything (because we're both guys) and we're not in love either. Without going into too much detail (unless it's appropriate/necessary), we basically have a kind of hardcore bdsm relationship that doesn't involve an awful lot of actual sex. We just provide eachother a special service that our girlfriends have never had any interest in. I've even asked my current gf about it (about engaging in some bdsm things) and she flat out said no. So it hasn't bothered me, doing all this behind my previous girlfriends' backs, but I'm starting to feel guilty about it now. I mean this is my first really serious relationship and I'm starting to wonder if I'm being a really nasty person for lying to her? I've asked the guy about it and he says that even if what we're doing is wrong, it'd be wronger to tell her about it - might mess her up psychologically or something... And honestly I like them both very much (the friend in a friendly way only) and I don't want to give either of them up... So what should I do?

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