According to a University of Iowa study, couples involved in "friends with benefits" situations are more likely to contract STDs, as they're more likely to have multiple partners:

According to Science Daily, the study, which surveyed the habits of 783 heterosexuals, shows that "being involved with a friend increased the likelihood of non-monogamy by 44 percent for women and 25 percent for men," and, interestingly enough, "respondents who got along with each other's parents were less likely to have multiple sex partners." [ScienceDaily]

[Image via Fashionably Geek.]