Nice question. I'd say yes, it is a sin to God. being gay or being a lesbian defies the entire purpose of our creation. God created Adam and Eve. Not Adam and Steve. Not Adele and Eve. Adam and Eve. He said go into the world and multiply. That's not something that two men or women can do. It's something that needs to be done with both parties present.
Also setting aside the original purpose of mankind on Earth, we can also see in the book of Genesis, God literally rained hell on a city because it was filled to the brim with homosexuals. I think his opinion on the matter was made perfectly clear with that.
Personally I'm not a fan of homosexuality but I also believe that a sin is a sin. None are bigger than others. So the same way God despises homosexuality, he probably also despises fornication, theft or even white lies.