Science Daily:

A new study of 434 young heterosexual couples ages 18-25 found that, in 40 percent of couples, only one partner says the couple agreed to be sexually exclusive. The other partner said there was no agreement.

Remember, kids: monogamy is natural and normal and humans are a naturally monogamous species and we evolved in sexually exclusive pairs over hundreds of thousands of years... which explains why we're so bad at it.

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