If you think about it at a basic historic level, when girls have sex they have babies. At some level guys don't want to provide for another man's babies and they want to know their kids are really theirs. The way we live now makes some of that unimportant and step kids are not unusual now but attitudes don't change super fast. I didn't remain a virgin until marriage (not even close lol) but I respect that lifestyle. I have been through so much dumb drama that wouldn't exist if I chose that path, although I also think it helps in some ways. Like you know when a guy is being weird or when how he acts is normal (or you know that about girls if you are male).