Then again, sigmund floyd invented a theory saying that all children experience incestious urges at a very young age. That is why boys are more likely to be mamma boys, and girls more likely to be *daddies girl*
This creates basic urges preventing homosexuality and jealousy among the same sex, causing sex that will produce offspring to happen more oftenly.

All of that is just a theory that noone wants proven though, so noone has ever tried.