Jealously is a complicated, tricky little thing that mostly sucks, can sometimes be productive, and according to science (and most rom coms) is experienced differently by different sexes. A recent study in Archives of Sexual Behavior shows that men and women get jealous about different things and the rationale probably comes from deeply rooted biological reasons.
The study set out to replicate an older finding which asserts that in heterosexual relationships, men are more likely to feel jealousy over sexual infidelity and women are more likely to feel jealousy over emotional infidelity; that is to say, women would rather their boyfriend or husband cheat by having sex with someone else than by falling in love with them and men would rather their girlfriend or wife fall in love with someone else over cheating. Researchers David Frederick and Melissa Fales took results from over 60,000 people that answered questions about relationships and dating and focused specifically on one set of questions that asked people to rate what they found most distressing: “You found out that your partner is having a sexual relationship with someone else (but has not fallen in love with this person)” or “You found out that your partner has fallen in love with someone else (but is not having a sexual relationship with this person).”
When they compiled the answers, Frederick and Fales found that 54 percent of straight men said they would be most distressed by their partners having sex with someone else but not falling in love with them, as opposed to the 35 percent of heterosexual woman, 32 percent gay of men, 34 percent of lesbian women, 30 percent of bisexual men, and 27 percent of bisexual women who answered the same thing. Straight men were the only demographic of people that had a majority indicate more jealousy over sexual infidelity, and everyone else indicated more jealousy over emotional infidelity.
So how is this rooted at all in science? From an evolutionary biology perspective, there's always a chance that the offspring a male is raising isn't his and investing time and energy to help someone else's DNA survive isn't ideal, which is why it would make sense that a man would prefer that his partner only have sex with him. Conversely, a woman can be entirely sure that an offspring is hers when she is the one gestating, and therefore has more of a demand for emotional energy from her male partner that will help during the pregnancy period and in the process of raising that child who will then go on to further perpetuate her genes. You get it.
While there's probably some influence biology has over these findings, it's really important to remember that biology is not destiny and in a world where humans are shaped so aggressively by their culture, it's impossible to make a distinction when it comes to the "why." The problem with only adhering to the the biological explanation, in this particular instance, is that it entails that our entire existence should be based around and aimed at procreation and reproduction and I think that can be really, really problematic.