If sex is not exclusive to marriage, then marriage is just a legally binding friendship. Sex is what is supposed to set marriage apart from all other forms of relationships, but now that casual sex is so common, marriage has no exclusive feature.
Sex is the most intimate thing two people can do. If you can repeatedly share that intimacy with strangers and walk away without a second thought, I don't believe you have capacity for love and bonding. You basically exist as an animal. I have never understood hookups.