The Institution of Marriage evolved over thousands of years, and has a number of vital, legitimate, logical, and efficacious properties.
It ensures that children are taken care of, that spouses are not discarded when they grow old, that violation of the marriage is sanctioned, if not punished, that the wealth of the marriage is preserved for the surviving spouse and children, and so on.
NONE of these is essential in a homosexual/lesbian relationship. Such couples cannot reproduce, neither of them need give up a career to nurture the children (in fact, homosexual couples have significantly higher household incomes, on average, than normal couples), when one of them abandons the marriage, the other is not financially devastated, when one of them dies provisions are easily made to provide for the survivor (assuming they are desired).
Indeed, monogamous ("married") male homosexuals are more a product of the AIDS pandemic than any desire for monogamy (the existence of which among homo's is essentially a pipe dream). The quest for the institutionalization of "Gay Marriage" is, more than anything else, a request for society's approval of a deviant lifestyle.
Still, we are all made in the image and likeness of God, and nobody knows the nature of anyone else's intimate relationships - unless it is advertised in some overt way. I know many heterosexual couples who have sex less often than they celebrate birthdays. Rumor has it that many lesbian couples are in the same boat, so to speak. So it is not rational or appropriate to assume that all homosexuals are sinners, even the "married" ones. And far be it from me to tell anyone what they may or shouldn't do when they are alone together.