Marriage used to be thought of...
...as between a man and a woman...of the same background.
The idea of a black and a white marrying used to cause civil unrest. So did the idea of two people marrying of different religions. Marrying a 'foreigner' would get people up in arms. This goes all the way back to marrying from different tribes. Marrying (having kids) outside your group upset the natural tendency of humans towards their 'own' kind. That tendency meant safety, security and social order which meant, or was intended towards, success in the game of life for the group.
This was all absolute common survival sense. It made sense biologically and it made sense as social policy.
As the human race became more and more successful, the biological needs and the social needs for purity lessened and have, frankly, fallen away. Tribes mingled. Races mingled. Entire peoples, nations and religions mingled.
If we take a casual glance, we see where the problems in the world are; groups desperately clinging to what they are vs. groups that are what they are; Jews vs. Muslims. Sunni's vs. Shiias.
Europe, for all their affected indifference to petty world affairs have their own divisions. Witness Muslim unrest in France and Great Britain. The only cool people are us, the US, and there's a reason for that; melting pot.
Marriage nowadays, for Americans, is about one and only one thing; social order.
Anyone wanna debate why a black and a white should not marry? A Jew and a Muslim (it happens)? A Catholic and a Baptist? An Italian and a Pole?
We can rationally limit the size of a union (2 members only) for practical purposes. We can not limit the gender of the members of the union on rational grounds. Gay marriage would serve the same intended purpose as 'straight' marriage; social order.
Concerns of 'yuck' factors over gender are as relevant as concerns over race and religion. These concerns are still legitimate but barely so. It is true that two people simply stand a better chance of a successful marriage if they have as much as possible going for them and that is because, as a practical matter, 'yuck' factors work against marriages. Yuck in this context is another word for prejudice. It is a consideration. It could get you killed in many nations. Here, people will look at you and say 'yuck'.
He does love them, he even cries when he has to send them all to Hell 
You mean I might be able to marry that little sheep that keeps winking at me. I do kinda have that tingley feeling toward her but I know it's just not right. You have given me hope though. 
Hey, I'm confused, your wife would object to Winn Dixie's proposal but you're willing to marry a sheep? 
but they need to keep it private. It's hard enought raising kids these days. I have to monitor all media very carefully to make sure they aren't washing some brainwashing pro-homo video or something.
Civil union maybe but NOT marriage in a church. Jesus says 
to homo's 
