Shows that homosexual individuals are abnormal beings. The subjects only common behavior (outside of behavior required to live) was homosexuality. This means that the only reason they are abnormal is because they are homosexuals. This by default makes homosexuality abnormal.
Unless you're trying to argue that homosexuality defines a person, it doesn't really sound like this makes a homosexual individual abnormal, just that homosexuality is abnormal behavior. To say that homosexuality is abnormal behavior isn't really saying much at all.
Left handedness is abnormal compared to right handedness, but we wouldn't say that left handed individuals are abnormal beings.
I imagine we all have at least one abnormal trait.