A few studies suggesting casual sex has consequences

My links from Google Scholar are dead for these two, but you should be able to pull them up quickly enough.

>Feldman SS, Cauffman E. Your cheatin’ heart: Attitudes, behaviors, and correlates of sexual betrayal in late adolescents. Journal of Research on Adolescence. 1999;9:227–252
>Treas J, Giesen D. Sexual infidelity among married and cohabiting Americans. Journal of Marriage & the Family. 2000;62:48–60.

Beyond that:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3407304/#!po=11.1386
>"Having more prior sex partners predicted a higher likelihood of future ESI"

http://onlinelibrary.wiley.com/doi/10.1111/j.1741-3737.2003.00444.x/abstract
>Teachman study correlating number of sexual/cohabitational partners with divorce, using 1995 National Survey of Family Growth