Most emerging adults engage in premarital sex. This has been true for awhile. What is new in this generation is that they have fully sexual relationships which they do not expect will lead to marriage.
Many think that getting broad sexual experience will help their later marriages. Some think that sex is no big deal. A few think that introducing sex early in a relationship will speed it along to true intimacy and love. They are all wrong.
The earlier sex is introduced in a relationship, the shorter it is likely to be.
Women tend to be emotionally hurt by sexual relationships that go nowhere, even when they think they won't care.
Men and women are scarred by broken relationships. The more broken relationships we have in our past, the harder it is to make a secure marriage in the future.
Interestingly, Regnerus and Uecker found that the intercourse itself was a positive factor in the quality of the relationship while it was happening, and even afterward. But that positive effect was outweighed by the negative of a broken relationship, and a broken sexual relationship was even more negative than a non-sexual one.