I always took it to mean that it wasn't THAT big a deal, as if he is trying trying to coax her into what he wants...not something one would be hung over is found out...
No idea, but there's always great/weird lines in Stones songs. Some makes more meaning than others. I always loved "You know marrying money is a full-time job, I don't need the aggrevation, I'm a lazy slob". So effin' awesome.
One more thing by the way, "hanging matter" ties in with the next line "it's no capital crime"...capital crime/capital punishment...again, he is saying to her that the proposed tryst is not that big a deal...
It's an old English colloquialism, "It's a hanging matter now", meaning the criminal would be put to death by hanging. In SCB Jagger uses it as a double entendre because he's obviously participating in statutory rape, which is illegal, and, I believe, WAS a hanging matter at one point. Of course the reference to phallic tumescence isn't lost in the translation, either.
"The wonder of Jimi Hendrix was that he could stand up at all he was so pumped full of drugs." Patsy, Patsy Stone