Dana, my friend and confidant, a Bowlie member and knower of much about Belle and Sebastian (whom I quoted previously), explains just what on earth that song is about:
There’s a lot of disagreement about what an arab strap is. As far as I can figure from what I’ve read, the etymology comes from restraining devices used on horses (Arabian horses, thus the name) during mating; the original Arab Strap was probably something like a cock ring. As such, there are cock rings out there called Arab Straps, but since there’s all this confusion about what one is, there are also strap-on dildos out there referred to as Arab Straps.
The song “The Boy With The Arab Strap” might be interpreted as being about a guy with some interesting taste in sex toys, but it’s also been interpreted as a dis on the singer of the band Arab Strap, which used to be on the same record label as Belle and Sebastian. I don’t know much about the rivalry, but you could probably find more info about it online.
Secretly Ironic 