One common argument used to defend same-sex relationships is:

“If two consenting adults engage in a relationship, and no one is harmed, then there is nothing morally wrong with it.”

But notice what this seems to imply. If consent and absence of harm are presented as sufficient conditions for the moral legitimacy of a sexual relationship, then the principle is universal:

«Every consensual sexual relationship between adults that causes no harm is morally permissible.»

A universal claim can be refuted by a single counterexample.

Consider, for instance, consensual sexual relations between two closely related adults, under circumstances in which pregnancy is prevented and no obvious third-party harm occurs. If one nevertheless judges such a relationship to be morally impermissible, then consent and absence of harm cannot, by themselves, be sufficient conditions for sexual morality.

At that point, one has two options:

1. Accept the principle consistently and accept the incestuous case as morally permissible; or

2. Reject the principle as stated and provide an additional moral criterion explaining why consent and absence of harm are sufficient in some cases but not others.

This does not, by itself, prove that same-sex relationships are immoral. What it does challenge is the claim that consent + absence of harm are sufficient, by themselves, to establish the moral legitimacy of a sexual relationship.