An individual's choice of sexual partner is of no concern to the government, as long as they are both consenting adults entering into a non-incestous relationship.
Indeed, by drawing a legal distinction between homosexuals and heterosexuals in the application of laws, you create a new classification of citizen.
So the result of your restriction is that We The People cannot, through our representatives, make any laws that embody this assumption. The law must treat committed homosexual relationships as of equal value to a man and woman who marry and procreate.
Sorry, it doesn't make sense to me.