Why not? If you are having a healthy pregnancy and are not high risk, there is absolutely no risk to the baby.
I've had high risk pregnancies, we were "supposed to" not have penetrative sex or have semen come in contact with my uterus (as semen is a prostaglandin and can trigger contractions in women who already have had pre term labor) nor was I supposed to orgasm. That wasn't going to work. I talked with my doctor, and we decided I could have gentle sex and take an aspirin afterward to keep the contractions down.
(DO NOT take aspirin during pregnancy unless directed by your OB or midwife. Using aspirin to treat pre term contractions is highly controversial. But, it worked for my type of preterm labor (until 32 weeks, where it could have caused problems) as mine was based on high prostaglandin release.)
I was able to carry my babies fairly to term. 38, 39 and 36 weeks each.