As a sex therapist I have had several guys mention this. I also did my doctoral thesis on how guys construct their sense of their own sexual orientation.

Such thoughts and fantasies are quite common and acted on, too. It isn't particularly sexual, as you say. I suspect that the natural affection between men gets inhibited by fear of being thought gay. When you've had a few drinks or smoked a joint or whatever some guys just want to make out with other guys.

It's a cultural taboo, particularly in the US. Here in Thailand where I live nowadays, men often hug and hold hands and stroke and kiss their male friends.