So you are making a lot of assumptions there. Sex is good for your heart, it is good exercise. It is good stress relief. If you want to use it only to have children, that is your choice. I choose differently, and that is my choice. Ain't freedom wonderful? I am perfectly happy. Would I be better off financially if I were married, no doubt. It is much cheaper to live as 2 instead of as one. However, I have enough and enjoy life.
Sure, you can get herpes and other things, but you also limit what you get by a huge amount by using condoms. Being married doesn't solve this because married people have sex outside of their marriage. They always have. This is nothing new. It does help, but isn't foolproof.
We can go on and on with these arguments but I doubt it will help. I believe in individual freedom.