Is it cheating if.........

You say your guys don't have anything to worry about because you're not going to become emotionally entangled. So is "cheating" in your book falling in love with someone else when you're in a committed relationship? The fact is that most men are pretty damn good at compartmentalising sex and love. So I guess you wouldn't mind if your boyfriend had an impulsive screw in the alley behind some bar just so long as there were no emotions involved, only lust, and he didn't fall in love with her?

You can try to justify and excuse it as much as you want, but a lesbian encounter counts as real sex. If sexual fidelity is one of the ground rules for your relationship and your friend's marriage, then you were cheating that first time. Since you've continued to have sexual encounters with your BFF, you're two-timing your boyfriend.

Personally, I think if a woman I was involved with had ever told me that such a thing had happened I'd be more intrigued and aroused than pissed off or hurt. Maybe that's the compartmentalising thing. But I'm not your boyfriend or her husband, and I have known guys who reacted extremely badly when they discovered their partner had had such an encounter. Some guys are so insecure that they react violently on hearing about such things.

I suggest you and your BFF should tread very carefully, and you might both want to spend some time thinking about and discussing what exactly your relationship is about. Regardless of what you say, it sounds like there is an emotional component, and you might want to think about where you want things to go from here.