Marriage lowers the male sex drive.

And all over the world, wives are saying...(Insert punchline here.)

From News.com.au:

BAD luck, dad. Scientists have found testosterone levels are lower in husbands and fathers than in bachelors.Carefree single men have the most testosterone - the male sex hormone which governs aggression and sexual appetite - while married men have less and those with the responsibility of children have the least.

A US study shows single men have twice as much as fathers of a similar age.

Experts believe the fluctuations could be nature's way of encouraging men in relationships to settle down.

Psychologists say it's sensible from an evolutionary point of view. Testosterone prompts males to seek out females and to mate.