I have no interest what goes on in their bedroom.
Do you care if a heterosexual couple marries just for the tax break?
That's a pretty deep legal question. It depends on the contractual obligations of the two parties, which varies by state - and the intent of the tax code.
For one thing - say two people marry for convenience for another reason, other than taxes, but as it turns out, they also get a tax break as a result. On the one hand, to take this break may be fraudulent - on the other hand, to tell the IRS you are not married when you are married may also be fraudulent. Its a catch 22