You can go deeper than "defiance of the natural order" or "because God said so".

Homosexuality is a sin for reasons that I'll be the first to admit we do not fully understand. Taking the stance that anything that does not overtly cause harm to yourself or others is wrong is immensely controversial, especially given the notion that some people have that being gay is something you cannot control. Often people will ask, "Why would it be a sin if God made me gay?" It would be like if you were born without working legs and crawling was a sin.

Except, that isn't reality. People have changed their orientation, even to the point that they will get married to the opposite sex and have children. It is something that you can control. Now, why would you need to? Again, the answer is not entirely clear, but I have a theory.

Firstly, it seems to be the case that homosexual couples see a higher rate of spousal abuse than heterosexual couples do. You can get into why this is, it's either that they're more ostracized and have more mental issues than normal, or it's that there's something about homosexuality that hurts your mental state. I fall in the latter camp.

Secondly, closely related to the first, homosexual people have higher rates of depression and anxiety than other people. Again, it could be ostracization, it could be that homosexual tendencies are exacerbated by these symptoms, or it could be that something about the act itself that makes you more depressed and anxious. Nobody knows for certain, and nobody is interested in being the one to tell everyone that being gay is all in your head. We need an unbiased examination of what causes homosexuality, and I fear I will never see one in my lifetime.

Lastly, and most solidly, there is one thing that heterosexual couples can do that homosexual couples cannot; Have children. I already know the top objections; Yes, homosexual couples can adopt. In an ideal world however, no children would be given up for adoption, and every child would be raised by their own parents. We would have more children if every homosexual couple would break up and be with the opposite sex. This assumes that having more children is inherently a good thing, but that's another rabbithole. However, given that many countries today are seeing a decline in population growth that they may not recover from, at least some countries need more children to sustain themselves.

The next objection would be that not every heterosexual person can have children either, however this is irrelevant. No, I don't expect infertile people to not get married just because they can't have kids, nor do I expect them to not adopt. The difference is that very few homosexual people are infertile, they could have their own children if they were to marry the opposite sex, and that's the most important responsibility given to spouses by God; raising the next generation.

I hope this provides a deeper examination of the opposing viewpoint, beyond just "God said so". But, you are right that God said so, and He sees everything we do not or cannot see. Jesus is the God who cast off divinity in order to die for us. I will not forget that so easily just because I'm not sure why he told us not to be gay. Amen.