Well that is indeed a scary scenario you outlined there. However that being said the basic reason for a sexual relationship is reproduction not personal satisfaction or pleasure. Now I will assume you know the basics of how babies are made. What exactly is the purpose of a homosexual sexual act outside of personal pleasure?
You also mentioned homosexuality is a naturally occuring part of human life. How so? The truth of the matter is up until the 1970s in the united states at least it was treated as it should be treated in that it was a mental disorder. That means that persons born with homosexual tendencies were treated as anyone else who has a gambit of mental disorders however what we find now is the opposite, with a society that promotes these tendencies instead of hinder it. Also when you talk of society having an impact you tend to downplay what that impact is. What I am referring too is not simple words or personal beliefs. What I am referring to is the outright hostility to normal functioning and natural relationships. I am forced against my wishes to adhere to the standards of the homosexual lobby. Stores like Target making gender neutral toys eliminating seperate sections. Transgender bathrooms. Redefinition of the term marriage. Demanding an artist such as a cake maker to conform to the will of a minority against their wishes. A college system and companies enforcing gender identity and promoting it while at the same time degrading what is natural. That is the kind of social impacts I am talking about.
Now to your last "Challenge" it is quite simple as I would use my earlier example of the very fact that it has absolutely no biological necesity. The problem however is that you say without using ethics and morals to make my argument. Well there is the crux of the problem. We have different places we get these things from. What is the moral and ethical standard for you? Oh right we use relativism. Do whatever make you happy am I right?