Most likely, long before any of us had a conscious original opinion about the morality of homosexuality we had a gut response of attraction or revulsion. This probably applies to almost everything, though some things may be so bland as to elicit no response either positive or negative.
Aesthetic value judgments are very basic. Often they come unbidden before we have any opportunity to evaluate them. They function largely beyond our control. I don't like beets. I can't give you a good reason why I don't like them, I just don't.
Just because aesthetic value judgments are often beyond our control does not mean they are necessarily hard-wired into us. Some tastes change over time. Also we are heavily influenced by our culture and environs. Western people tend to hear and respond more favorably to music written in a western style. Music written in the pentatonic scale (which is predominant in Asia) sounds alien and unpleasant to many westerners.
Most aesthetic value judgments are morally neutral. It does not matter if you prefer Picasso to Seurat. I can question your taste, but it is no reflection on your character. However, not all such judgments are morally indifferent. Consider your gut response to seeing a man in Arab dress on the airplane with you. Or have you ever crossed the street to avoid walking near someone who appeared unsavory? Aesthetics are an insidious and difficult to overcome part of racism and discrimination of all kinds.
We are all familiar with the popular proverb "don't judge a book by its cover." Yet we also know very well that "first impressions matter." Like it or not our aesthetic values influence our moral judgments, usually unduly.
Of course, not all aesthetic judgments are a bad influence on moral decisions. There is a reason why people naturally recoil at images of violence and gore. There is a reason why a psychologically normal person is moved with pity at pictures of other human beings suffering. These deep seated aesthetic values are powerful assets that give us a head start in the empathy game. They hinder us from committing acts of violence and give us a push toward acts of altruism. But they are not a replacement for moral reasoning. If an aversion to blood prevents a doctor from being good at her work it is a hindrance not a benefit. If pity for the suffering of others is condescending it will be counterproductive.
When it comes to homosexuality aesthetics are particularly relevant, of course, because part of sexual orientation is an aesthetic value judgment. I find women sexy. Some more than others, of course, but almost any woman is more sexually attractive to me than a man. This aesthetic value isn't something I chose and it comes prior to any conscious evaluation I have made about the relative qualities of men and women. Because we naturally empathize (for aesthetic reasons) with people similar to ourselves I find the sight of heterosexual couples kissing to be more attractive than images of homosexual couples kissing. Many people are in fact repulsed by the sight of something they can't relate to, thus the sight of two men kissing makes many heterosexual men uncomfortable.
It is the unfortunate truth that many people who say they regard homosexuality as a sin have no better reason than "it's icky." This kind of primitive value judgment feeds right into Natural Law arguments (which I will cover in a later post). People assume their discomfort is some kind of proof that homosexuality is unnatural when it is no such thing, anymore than a dislike of beets proves that beets are unnatural (though they really might be!).
In making moral judgments every person has the responsibility to evaluate themselves and be sure that their aesthetic biases are not influencing their opinion. If your reason for declaring homosexuality sin is at any level because it grosses you out then the only thing to say is "get over it!"
Though we may not have control over our initial aesthetic values we can train ourselves to appreciate beauty which we previously did not see. I recommend spending time with an articulate and expressive homosexual friend who can explain their attraction. Expose yourself to images of homosexual expressions of affection - even better if they are real life people. Get to know them. Hear their stories. Share jokes with them. Try to see things from their perspective. Imagine, I mean really imagine, for yourself what it would be like to kiss someone of the same gender. How can you say you wouldn't like it if you haven't tried it? Tastes can change. I once despised asparagus, but I just didn't know what I was missing.
Doubtless that sounds scandalous to some people, but consider Caravaggio's filthy swarthy saints, or Michelangelo's nude David, or our beaten and abused savior on the cross... what is scandalous to you may be a glorious work of art to another person.