And to clarify, it's not always because of ill intent of the partner. Depending on preexisting issues and new conditions caused by a pregnancy, it's not uncommon to get an advice from doctor (or even a written advice from the hospital) to refrain from sexual activity. This makes the partner a much more interesting target for aforementioned sites.

And for more clarity, this can swing the other way, too. Sometimes pregnant women want sex more than normal, which can be totally fine/healthy depending on the health of the mother and baby. Hormones are extremely powerful and affect people differently.