The term is perhaps a bit out of fashion. We heard it first in Cairns in the mid nineties at a gay holiday resort. This gay resort thing was a bit intimidating for us. But we coped. As you gay people would guess or already know, the place was full of queens with attitude. That is ok, we have each other and we can bitch with the best of them.
One has this terrible confidence when one has a long term boyfriend. It is a bit rare in gay circles, so if you are not respected or treated dismissively, you can shrug your shoulders, not care, fuck you, we're off home.....together, just like we have done for umpteen years. This is absolutely no help when you meet other long term partners though. Given we had to endure a couple last night, it is at the forefront of my mind.
Better get focused. The phrase is, 'I'd do him', meaning you find someone is sexually acceptable. It paints an image for those who are not gay, of you being a sexually assertive and an aggressive penetrator.
Not so. 'I'd do him', does not necessarily mean you wish to have him spredeagled on the the bed and violate his orrifices with your manliness. It could just mean that you would like to sit on his appendage while he slaps your face with one hand, squeezes your testes with the other, bites hard onto your nipples and calls you a cock sucking whore.
As I said, it is an older term, but do not make a judgement if you hear someone say 'I'd do him'. It is a catch all phrase.