Do you regret not having sex with someone you wanted to have sex with? Do you regret having sex with someone who didn't turn out to be who you expected? What makes you jealous when it comes to dating and sex? These answers, research shows, depend largely on whether you are male or female.
Evolutionary psychologists argue that emotions exist because they serve specific roles in helping humans reproduce and pass on their genes. If an emotion didn't have a useful purpose, then we would not feel that emotion; it would not exist.
From an evolutionary psychology perspective, women have to be more selective in who they choose to mate with, because child bearing is a bigger physical, emotional and time investment for them. This is necessitated by biology; men can, theoretically, impregnate several women in one day if they would want to, and they pretty much can their whole lifetime. Women cannot—as if you needed me to tell you this.
From this perspective, women should have stronger responses to emotional infidelity, because it signals that the partner is not committed to the relationship, and by extension, will be less likely to provide for her offspring. Men, in contrast, should rue missed opportunities for sex, because these are missed chances to reproduce.