Your suggestion that marriage is simply about sex only supports the idea of same sex marriage-equality. It's a shallow view, IMO, and it plays into the hands of those that want to devalue marriage.
We're not talking about just sex. We're talking about a complicated relationship between people and their relationship to the rest of society.
Proper family formation is essential for the healthy continuation of society and civilization. A special social recognition and support of those people that wish to begin families is important.
Not complicated but complex. And your premise is 100% correct. Lose this and the foundations of our society are transformed. We are a tolerant people but we have had enough transformation.