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.
Research has tested the role of regret in sexual activity. This body of research shows that men are more likely than women to regret errors of ommission, that is, missed opportunities to have sex with a desired partner. In contrast, women are more likely than men to regret errors of commission, that is, times they had sex with a partner and the relationship did not work out as planned.
In sum, casual sex is likely to cause more regret for women than men, and men are more likely than women to regret missed sexual opportunities.
There also is a wide body of evidence indicating that women have more jealousy towards emotional infidelity than men, but that men have more jealousy towards sexual infidelity than women.
This is not to say that both genders aren't distressed by both, or that there are not exceptions. Not all men and women fall into these categories, but on average, men and women differ in their responses to sexual and emotional infidelity.
Basically, women respond more negatively to emotional infidelity than men do, and men respond more negatively to sexual infidelity than women do.