Very well. I find it incredibly difficult to believe that such short-term sex can contribute to a meaningful relationship. If you took a society, cloned it, and then had the first society continue like ours does, and the second continue without premarital sex, I believe you would find a much greater level of net happiness among the second society. Of course, this point is entirely untestable, so we're just going to have to stick with what we each think about it.
You're okay with 2 unwanted pregnancies for every 8300 acts of sex using condoms? Assuming "95% of Americans have premarital sex", then that's 74,458 unwanted pregnancies. Geez. 74,458 lives that have a high probability of being raised in an environment without the proper resources for a positive development (again, I can cite some facts about this if you like). Only 43,400 people are killed in car accidents each year. You are not only affecting your own life (as in a risky act like sky diving or free climbing), you are affecting someone else's too. If you're okay with that, then fine, but I am very surprised.
Gay people and those are in their 70s cannot accidentally have a child.
And those people who drink and drive, and text while driving, are WAY more impractical than those who have premarital sex and are MUCH more damaging to society. But it doesn't make something that's somewhat impractical any more practical by comparing it to something vastly more impractical.
Then I have no longer have a case that it is impractical -- taking the assumption that you have given that sex is a highly valuable relationship-building tool (which I expressed my own thoughts on in the first point above). In fact, I would be all for free vasectomies and tubal ligations for those who want it.
And... ?
Again, if you're fine with trading the well-being of 74,458 lives for some quick sexual gratification, then so be it. And again, I am surprised.
"Not necessarily" -- those are the key words. We can still substitute "but with high probability". If a person is REALLY mature enough to commit to a lifelong relationship (and there are VERY VERY few that can do this at a young age), then yeah, they are much more prepared than that same 40 year old who, randomly selected from the population, is likely to have tried drugs, been arrested at some point in their life, had a divorce, and who's kids turn out only mediocre in school, with mediocre jobs, and a mediocre happiness. (Yes, mediocre is the appropriate term for an average person).