A defect is defined in genetics as anything that is detrimental to the survival of a species. I'd say the inability (or rather, unwillingness) to reproduce is an obvious hindrance to the survival of any species.

To everyone else, except Odlov, who brought up some good points on the fiscal and social benefits of marriage, take love and affection out of the equation and then formulate a decent argument.

@Odlov,

While it's true that denying certain benefits to people based on something as seemingly arbitrary seems "unethical", think of it as a negative incentive of something that society as a whole still very much frowns upon. And for whoever mentioned the unconstitutionality of anti-homosexual laws, do you really think the law treats everyone equally? As much as some would like to think that the law views everything objectively and treats everyone with a general demeanour of egalitarianism, if everyone was equal in all respects, there wouldn't really be much of a need for those laws in the first place, would there?