Chris Masterjohn pointed out that my last semen post pointed towards a biological bolster for monogamy in this modern age. Here is another. I'm sorry but this post is less fun than the last one about semen, but I promise another one that is less alarming in the future, though I have a post in the queue on birth control that's rather unfun.
So humans are one of the few species that engages in non-reproductive sex. We have sex when the woman isn't pregnant (and between people who can't get pregnant), but also when the woman is already pregnant. Biologically this seems a little pointless. But this paper(PDF) sheds light on reasons why it might be beneficial, as long as the semen is from the man who is the father of the baby.
Preeclampsia is one of the few true dangers to pregnant women in the first world. It causes severe hypertension and can cause the loss of the baby, the mother, or both. The death rate is 790 maternal deaths per 100,000 live births.
The prospective causes of it are complex and I'm not going to talk about all of them. Evolutionarily some scientists speculate it is caused by the nutritional hunger of a developing baby's brain, which is very high compared to our forebearer species. Other species don't get it at the fairly high rate we do. The paper discusses the fact that evolutionarily speaking, paternal investment improves survivability. Sex at Dawn argues against this using examples of some modern tribal societies where children are raised communally, but it's impossible to know whether this was the state of things in the paleolithic and either way there is good evidence that evolution has continued since then.
Several studies, particularly on babies conceived with donor semen, show higher risk of preeclampsia in pregnancies with non-paternal semen exposure. If you have a second pregnancy and the father is different from the first, it seems risk for preeclampsia is also higher. Interestingly, exposure to consistent and familiar semen (a la in a monogamous relationship) seems to increase odds of getting pregnant.
What about just not having sex during pregnancy? Don't do that! That increases risk of preeclampsia as well! You might as well have some oral sex, as that is also shown to reduce odds of preeclampsia in some studies.
Why does this happen? Perhaps it's an evolutionary response to the dangers of carrying an unwanted pregnancy (from a rape for example) or perhaps to the benefits of paternal investment. Either way, it's very interesting and supports the idea that if you want to have a healthy baby it might be better to be married or in a monogamous relationship.