First, I'm straight, and even though I support same sex marriage, I believe we should respect people's opinion who is against it, but your reason to be against that is not a good reason.
Family unit?
Do you know in many countries the men were allowed to have a few wives until early 20th century, I'm not talking about just middle east, that was allowed in many countries.
Do you know in most countries, choosing own spouse was allowed from 19th century, do you know until then in most country include Europe, people didn't have freedom to choose who they could marry to? It was always "arranged marriage" by parents.
How to make family has changed through out the history.
Look around, Can you see how many single mum/single dad family? Can you see how many kids have two family because of parent's divorce? Can you see how many straight people give a birth without marriage?
Family means blood to some people or family means people who love each other unconditionally to some people, and I don't think the meaning of family will change because of same sex marriage.