Gay unions cannot produce children. No amount of wishing it weren't so will change that fact. Nature says no to gay families.
Most "gay" people have normal sex sometime in their lives, and many get married to a member of the opposite sex and have children the usual way.
Then they might "discover" they are "gay" or "come out" and then get a divorce. The notorious Anglican bishop you've been hearing about was married and had two daughters, for example.
With "gay" marriage, it changes the whole custody climate. Who has a stabler home, the single parent or the married one? Get it?
To say nothing of adoption and reproductive technologies.
This issue involves children directly, not through some chain of reasoning about slippery slopes.