The big difference between sex for money and sex for free is that sex for money usually costs a lot less.

The line is a sardonic inversion of moralistic assumptions about prostitution. By treating “sex for free” (within romance or marriage) as potentially more expensive than paid sex, it reframes cost as emotional, social, and economic—courtship, gifts, obligations, jealousy, and long-term entanglements. The joke depends on blunt, transactional language to puncture hypocrisy: society condemns explicit payment while often tolerating implicit exchange. Attributed to Brendan Behan, it fits his public persona for bawdy, anti-pious wit, using humor to expose how respectability can mask its own forms of bargaining and exploitation.