First, it’s wrong. Pregnant women are meant to be mothers in training, chaste and committed to motherhood. So for them to be explicitly sexual is hot. It is dirty.
Second, it’s known that being pregnant can sometimes bring a period of increased sexual interest, physically. And there is nothing hotter than an uncontrollably horny person.
Third, being pregnant is a sign that I am taken- a pretty powerful one. And there is just something about wanting what someone else has or finds desirable i.e. ME!
Fourth, a pregnant woman’s body changes often. And the dramatic changes in such a short time are something to be enjoyed.