Can anyone give me a VALID reason why gay marriage should be illegal?

I'm a Catholic and personally, I have many friends who are s. I don't believe it's right that they can't get married for the reason that they were born that way. It's not a choice, it's what a person is and God loves everybody. That's what I've been told and that's the God I follow. I have three children and would never love them less if they're uals and would want what makes them happy. I'm just a human being and I love my children so much. How can a Perfect God not love all His children the way they were born? Deep in my heart, I know he loves everybody who loves Him, even the ones who don't, but they need to change and love Him and trust Him, that's all.