Sexuality of Unborn Male Child

A nice sexual rumour that I heard in the ticket queue at Waterloo Station on the 1st August 2005:

Two white women in their early 20s, either students or recent graduates, were discussing families and gay children. One mentioned a belief that if you have unprotected sex whilst pregnant with a boy, he will grow up to be gay. Neither of the women believed this.

I had not heard it before or since, and I wondered how widespread it might be.