Sex is for husband and wife to have kids.
It's so that believers in God can be blessed for doing missionary work they wouldn't do otherwise. They bring their kids to faith in God, and to salvation. Since this is the greatest job that Jesus did and wants done, God blesses them for getting people saved, even if it's only their kids.
Yep. God literally makes an excuse for Himself to bless people extra by doing this for them. Unfortunately, people mess it all up.

sex can also be a sport, a party, a joy.
islam allows later, chatolicism bans them all and allows sex only for reproduction.
i am ok with both, but i value also both. there are situation one is better than the other and vice versa
Sex for kids only! That way there are more kids, and when they grow, more income taxes.