My problem isn't even with EVERYONE who has premarital sex. It is with the setup.
I propose to you what SHOULD happen, and I will contrast it with what DOES happen.
What SHOULD happen: A couple marry young (Teen years). They go through everything together. First kiss, first sex, high school, college, first job, first time travelling together. They form an unbreakable bond. They stick together through thick and thin. Eventually, they raise kids and a good, wholesome household. Because people marry young, hypergamy isn't NEARLY as bad.
What DOES happen: People get into "LTRs" only for one person to leave. Each person's ability to pair bond and ever love the same way is severely damaged. Out of fear and desperation (And hedonism, in some cases), people just start throwing themselves around. At the end of the day, they're only marrying partner #7+ when they're completely jaded. Others are left out due to hypergamy (Really not a bad thing in this case). They're literally just "settling", hell, that's LITERALLY what they call it. The marriage means nothing, the home life is garbage for the kids, the people live on miserably or get divorced. Lots of coping with drugs and alcohol from the teen years on. It's complete garbage.
This whole "I need to 'date' around to see what I like" concept is nonsense. Do people REALLY think that building and breaking bonds, living wondering about what could have been, and annihilating those who love you is superior to building an unbreakable bond with one person? Unfortunately, many people think so. And they pay the price every day in their regret and blind stumbling.