The story has it that it was an 'easy access' style for bum-sex in jail.
The jail bit made it to the outside world as street-cool, the other detail did not.

Or alternatively, everyone is a lot more relaxed about homosexuality than was assumed?
(BeatsWork, Thu 1 Mar 2012, 5:03, 2 replies, latest was 13 years ago)

^
Exactly this
(wehttamman via B3ta for microwave, Thu 1 Mar 2012, 5:57, Reply)

There's a couple of other theories, too:
1. That it's from when they'd take your belt off you when you went to jail - I've been to jail - don't mess.

2. That you always get your older, bigger brother's hand-me-downs - we're from the rough side of town and I've got a bigger brother - don't mess.
( Je suis un vagabond is an unfunny, up your own arse middle class knob, Thu 1 Mar 2012, 11:38, Reply)

3. I'm such a rebel,
I've told mummy that I'm old enough to dress myself in the mornings, but haven't got the hang of it yet.
( monster munch person, man, woman... camera... TV?, Thu 1 Mar 2012, 12:08, Reply)

Well - yes. That's the actual reason.
I'm talking about the origins.
( Je suis un vagabond is an unfunny, up your own arse middle class knob, Thu 1 Mar 2012, 12:29, Reply)