Confused children who have two mommy's or daddy's.
OK...that's one.
Our anatomy is not designed to accommodate same sex, sex.
This is an opinion, not a fact. Our human anatomy is far more versatile and agile than you give it credit for being.
I'm sure that there are plenty more, but for me God's reasons are enough.
Unfortunately, in your response, you've only provided one reason "against" gay marriage. Not homosexuality in general - just gay marriage.
And using god as a reason simply does not make an argument. However, if it suffices for you - that should be enough - for you.
Just don't try to foist your belief system on those who do not believe as you do. You're likely to run into some very stiff opposition!