I'm not 100% sure, but I think crabs need an area with hair to survive.
I also think if you have a lot of body hair, such as hairy chest etc, they can move to other parts.
But I do know that a cream available from the Pharmacy will get rid of the problem!
Unfortunately rubbing the cream all over your SO, does not erase the memory of what you have done!
Maybe someone should invent such a cream, you would quickly become the richest man on the planet.