I had sex with 3 of my long-term boyfriends. Two of those relationships failed (obviously, or there wouldn't be 3). The first was in high school, I was 17, young, and still very naive and did not know then what I know now.
The second was my first boyfriend in college. We were together a year. But we had sex after the first month. Then after a few months, the sex suddenly stopped because he felt guilty (religion had a major effect on this). After that, our relationship slowly went downhill until it finally crashed and he and I are no longer in contact and have nothing nice to say about each other.
I am currently with the third, and we too started having sex after the first month. It has been a year and three months since we've been together and we are expecting a baby this summer. We have been having some difficulties lately, but we are in the process of working them out and working through them so that we can be a family.
My opinion on the matter is, in the long run, it is better to wait until after you're married. But, these days, there really is no stopping it, so I cannot judge anyone for choosing not to wait, especially since I chose not to wait, too. That being said, I also believe that sex CAN be beneficial in a relationship. It is just a level of intimacy that a couple has to judge for themselves and decide what is right for them.