Tim Keller engages in excellent cultural (as opposed to political) defense of marriage, at Google's NY office.
Check it out!
It struck me listening, how much of cultural defense (apologetics) is simply explanation of the truth.
13:00 - there's no one out there perfect for you
20:50 - sanctification in marriage, toward glory
25:30 - the secret: loving when you aren't getting love
54:00 - what sex does in a marriage
56:30 - on gay marriage