Sex in and of itself is not irresponsible. It's how we do it. And frankly, if I'm in the mood for a fling, I will have one provided we're both on the same page that it's to serve a physical need and that we take care not to put the other at risk.
And if I were to wait for marriage before having sex, I'd be celibate for the rest of my life because I just don't believe in putting a contract on love. Real proof of love is loyalty, commitment and dedication and respect for one another, not a piece of paper or a ring. Up to 75% of married men cheat and up to 50% of married women cheat. So, to say marriage = responsible sex is not really true.
It's up to people themselves to be responsible. Marriage does not automate responsible sex.