What is the difference between a father and a mother?

This is the fundamental conflict here. Liberals look at marriage and see something people do to be happy. Telling some people they can't marry and be happy is just mean! They fail to see the deeper consequences of messing with tradition.

Yes, I think that's exactly right. To elaborate, I see the attitude of not wanting to mess with tradition as being the same attitude that used to make people say "if God wanted us to fly he would have given us wings".

Perhaps messing with tradition is risky, as was trying to fly. But I'm ok with that.

So what are the deeper consequences of allowing homosexual couples to legally marry?

It devalues real marriage. The consequences will be much the same as past developments that devalued marriage as an institution and an oath, for examples no-fault divorce and much public welfare. One can reasonably anticipate more out of wedlock births and more divorce.

Just to be clear, you're seriously arguing that homosexual marriage will lead to more out of wedlock births and divorce among homosexual couples?