Dan Slater Atlantic has a good article about a sad phenomenon: online dating makes some relationships less stable and less likely to lead to marriage.
One of the reasons for this instability is that online relationships tend to quickly lead to sex. As Mark Regnerus and Jeremy Uecker found in Premarital Sex in America, the earlier sex is introduced into a relationship, the shorter the relationship is likely to be.
This leads me to think there is a market for a new kind of dating site: one of the ground rules would be "no sex till marriage." Or at least until she has a ring and a date.
Such a site would separate the let's-have-sex-and-see-what-happens online daters from the I-am-here-for-lifelong-marriage online daters - to the mutual benefit of both groups.