This is a very difficult question for me. I was raised in an incredibly liberal area of California, and therefore when I converted to Catholicism, it was extremely difficult for me to wrap my head around the concept of homosexuality being sinful.
As far as I can tell, it’s something that you can’t really understand without faith. It doesn’t make sense to non-Catholics, why God would give us something that is bad. It seems unbalanced to non-Catholics that some people would be “required” to live a life of chastity. Unless you believe in what the Catholics believe in, it’s difficult to understand. I can tell a person that it’s not unfair because God will treat us all exactly equally in the end, but then they will ask how I can know that for sure. I will then respond that we have to trust God, because we know Him to be fair and just in all cases, but because they have no trust of God, the argument makes no sense to them. Or when I tell them that because God will never tempt us more than we can handle, those who are given the temptation of homosexuality are only those who are strong enough to be able to stay in God’s grace; but it’s not logical to them that that could happen, because to an atheist these characteristics are nothing but random gene sequences, and to them it’s impossible that the two traits of strong temptation and strong faith will always go together. When I tell them that we simply must trust God, they cannot understand, because they do not have faith in God.
I’ve been a hard-core liberal Christian. I have made all the arguments about why it is nonsensical for religion to say homosexual lifestyle is sinful. I couldn’t find any logic in the arguments from people such as Catholics, until I actually became Catholic. Once I believed in the faith, I realized that I had been dead-wrong about an issue that seemed to so natural and obvious. I really do that in order for a person to truly understand certain things, they need a certain amount of knowledge of the faith on which they are based. It doesn’t mean they need to be Catholic, necessarily, but unless they have knowledge of the faith, at least of where we are coming from, you can’t just explain it to them in one shot.
So mostly, if I am asked the question, I will typically decline to respond on the grounds that I doubt they will be willing to see the logic behind faith-based arguements. For the most part, they tend to agree.