When discussing whether Homosexuality is morally right or morally wrong, I've always argued that if we allow homosexuality then we would have to allow incest as well. Before arriving to this conclusion I first looked at the various arguments defending homosexuality which mainly consisted of the following: 1) It's consensual (with the exception of rape); 2) It doesn't harm anyone; and 3) It's a matter of love (i.e., we should have the right to be with whomever we love). Now my reasoning is this: All three of those arguments could be used to defend incest! Imagine a father who becomes sexually involved with his 20-year old daughter. Both would be consenting, they are not harming anyone, and they presumably have some type of attraction towards each other. My question is if my argument is a good one or am I missing something?

First, there's a difference between showing that an argument for permitting homosexuality is bad and showing that homosexuality shouldn't be permitted. To show the latter, you need an argument to that very conclusion; it won't do to show that some argument for permitting homosexuality is actually a bad argument. Refuting an argument in support of X is different from giving an argument for not-X. Second, I'm not sure you succeed in even showing that the argument you consider for permitting homosexuality is a bad one. You want to say that if the argument were correct then it would also permit incest; since the latter shouldn't be permitted, something must be wrong with the argument. But I'm not sure I agree that the argument you consider about homosexuality really would also apply to incest. That's because I think that incestuous relationships do often lead to psychic harm for one or both of the individuals involved (as opposed to homosexual relationship, which don't lead to such harm – at...