I've seen the "gays can't reproduce" argument against same sex marriage posted here many many times. The suggested legislation merely follows that claim to its logical conclusion. It's obvoiusly not intended as an actual ban - merely as a means to point out the absurdity of using the reproduction argument against homosexuals.

__________________
A nation of slaves is always prepared to applaud the clemency of their master who, in the abuse of absolute power, does not proceed to the utmost extremes of injustice and oppression.
Edward Gibbon