They claim it is a "genetic" trait when actually the larger Sex Positive Agenda seeks to end all moral judgment over sexual pairings regardless of sex, age, relation, marital status, number, or species of partner(s).
As humans, we can choose to act on our desires or not to. Sex Positive (and certianly Homosexual) advocates claim it is unhealthy to supress a sexual desire. This is why they also oppose abstinence (NOT because "it doesn't work).
A man may or may not be able to get erect looking at a woman (and even heterosexuals are not hot for every woman, see Helen Thomas) but he certainly has a choice as to whether he acts on a sexual impulse to hump something/one. If factors like a wedding ring, boyfriend/girlfriend, age can come into play and make him reconsider acting on that lust, then so can the sex of that object of desire.
If you scratch deep enough in the writings of those who are still angry at McCarthy, you will find them saying "it wasn't a crime to be a Communist". While the familiar mantra is "they weren't really communists", the truth was they saw nothing wrong in it but were too afraid to take THAT stand.