This question crossed my mind a few days ago, and I'm still not sure. I can imagine three scenarios.
In the first one, it's invented by a guy. I mean, of course only a guy would think up such a thing.
Then again, I can see some enterprising young woman, perhaps intent upon becoming betrothed to one of the wealthier men in her village, tribe, whatever, thinking, "I know how I can win this shit, easy!"
Then again again, it could have been a gay dude that invented it. This is entirely possible. One guy thinks, "Man, I wish I could get this thing in my mouth.. Hey, I've got an idea!!" Then he finds another dude and gets him to agree to "do me if I do you."
What do you think, YaHooka?
The Rev