Morality. And enlightened (long-term) self-interest.

(I thought Slashdot was the one of the places where people realized this.)

Though of course, not everyone who can't get a real date is a disgusting old lard-arse.

An interesting angle on this debate is one that has received coverage in the UK recently: disabled people using prostitutes. There was a sympathetic (but pay walled) article in the Sunday Times recently:

"Some disabled people are cut off not only from sex but from any kind of gentle touch. People whose disabilities have prevented others ever being attracted to them are painfully conscious of the raw judgments of the sexual marketplace. ... It is impossible to read some of the joyous, grateful accounts of people who have just had sex for the first time without feeling that simplistic judgments about the evils of prostitution are an inadequate response to this need. A man who cannot talk wrote thankfully about his wonderment at not being made to feel awkward or stupid; another said his escort was gentle, intuitive and trying hard to make the whole experience special. In this surprising world, their pride is often matched by the prostitutes’ agencies, some of which advertise themselves as “being able to provide a genuine unhurried caring service for our disabled clients”."