God permits many things in this fallen world that are not what He wants. God's intent and ideal in regard to marriage has always been one man and one woman for life. There were probably reasons why polygamy was more common in the OT days, but there are a number of New Testament scriptures on marriage, and those passages always use the words 'husband' and 'wife' in the singular, not plural. Plus we know that Jesus reaffirmed God's original design for marriage. (Matthew 19:4-6)
As for dietary rules (on what is kosher) it is my understanding that those only applied to the Israelites. Those laws were ceremonial and temporal, which is completely different than moral laws which are universal and unchanging.
So if what you're implying is that homosexuality falls under the same category as defunct ceremonial laws, that is absolutely incorrect. Homosexuality is condemned in both the Old AND New Testaments, and it is a moral issue, as opposed to civil or ceremonial.
That was a specific command to a specific group of people at a specific time, and it has nothing to do with God's universal unchanging moral laws. There were reasons for that (that's a topic in and of itself) but it is wrong to try to conflate a command meant only for a specific group / place / day in history to a general unchanging universal moral command.
That's where you go wrong, your belief that gays are born that way. There is no 'gay gene' and it is
not how God created them to be. I believe there might be a small percentage who are born with a
propensity to be homosexual (like there are people genetically predisposed to alcoholism) but things like that are a result of this fallen world, and not at all the same as natural things like eye color, gender, etc. And as others have already said, even that is not a justification. By that logic, we could excuse just about anything, and there will certainly come a day when pedophiles will say "I was born this way."
As for what causes it, IMO there are a number of possible reasons, including things that happen in one's early childhood. I've noticed that
a lot of homosexuals were sexually abused or molested in their childhood. I think there are other factors too, but this is getting too long and wordy.
You don't have to "condemn" your friend, but you also shouldn't mislead him by agreeing with the worldly lie that there's nothing wrong with homosexuality. At the end of the day, our standard is not what matters, only God's standard matters. And remember, with God all things are possible. So there is hope for everyone, but not if they continue to believe the lies this world bombards them with.