let's be honest: how many relationships which you start in your early 20ties lead to marriage? i bet not a lot or at least not a lot which do not get divorced anytime soon so why date and eventually hurt someone or being hurt if he/she or you develop feelings and want a commited relationship? from a logical point of view one night stands make way more sense for me anyone else thinks like that?
I feel like you date in your early 20's to find out about yourself (when being involved with someone else) and to find out what exactly you want or what fits with you when your truly looking to settle down. Like what some of the other people said, if your fortunate enough to meet your future wife/husband then great, but if it doesn't work out that way I don't think it should be looked at as a failure or waste of time because your at least learning more about yourself and what to avoid or go after when looking for someone to settle down with.
I know quite a fair amount of people who met their marriage partners in their early 20's. I even have friends that are planning on getting married soon. It's definitely possible to meet your life partner in your early 20's. Besides there's no time limit on love. You can meet the love of your life at any age and have it work out. The fact that you meet at a certain age doesn't prevent true love from working out.
I met my ex-wife when I was 27. She was my first serious relationship. I am soon to be 35 and now going through a divorce. Truth is, I wish I had a girlfriend earlier. Because I would've learned some things that maybe would have made me reconsider getting married to that women.
Now I started dating again. No to find love (yes if it happens I am not closed-minded to it) but to learn about myself. Truth is, I have met many interesting women since I started dating. And even if not all dating experiences were great, each one of them thought me something about myself. Being hurt now and then is the price you pay to go out in the world. Finding your one true love might be the ultimate goal, but the life experience you get along the way is the real reward.
It's free experience. If you only get serious for the first time when it finally counts you might make a lot of mistakes you could have avoided if you had already been in the same situation before. It's a lot easier to make marriage work if you had some relationships that didn't.