When a staight male puts his reproductive organs into the reproductive organs of a straight female that behavior needs to be regulated.
When a straight male or female unites his or her reproductive organs with those of another of the same gender that behavior does not need to be regulated as no third party is going to be, or likely to potentially, effected.
Likewise the contracts that straights engage in when they establish unions where they will habitually be uniting their organs need to be regulated whereas the contracts of gays do not. No third party is going to be effected nor is a third party at all likely to be effected. There will be no children, there will be no motherhood created, there will be no inheritance to pass on to the non-created children, there will be no need for divorce court to make sure that the children are protected when they divorce, there will be no abandoned children, there will be no grandmothers made, there will be no grandfathers made, no aunts, no uncles, etc.