I don't know how common it is, but yes it is normal for male friends to have sex. Two close friends who have sexual needs and no mate to share it with can easily find a comfortable partner with each other. It's called friends-with-benefits sex, and/or may simply be experimentation. I've had sex a few times with a male friend of mine, and despite it being fun and feeling great, I'm still as heterosexual as I always was because I feel no emotional/romantic or physical/sexual attraction to guys (my friend is bisexual). It started out as talk, and then our first time was experimentation to see what it would be like, and subsequent times was friends-with-benefits. The key part was talking about it. We talked about it for several years before we actually did anything physical, and were both completely comfortable to move our friendship in that direction. FWB is not something to be jumped into without a lot of discussion and understanding.
Assuming there's any truth to your anonymously posted story, what you should do is talk to your friend honestly about what happened and what your feelings are about it. You don't need to keep reassuring yourselves and each other how much you love women. Being heterosexual and loving women doesn't mean you don't, can't, or shouldn't enjoy sex with each other when that provides both of you a pleasurable outlet. But also, since you don't want to lose your best friend, tell him that as well, that the friendship means much more to you than sex, and that if sexual stuff can't be a comfortable part of the friendship then it shouldn't have any part of the friendship. You can't undo what happened but the two of you can decide how to move forward and whether or not to let it happen again.