Well said LimeSlice
Pregnant women are sexy.
You used to be his wife/partner and lover, and that’s pretty good but now you are growing his baby. You will soon be the mother of his child. He can watch the baby growing, see the baby moving, and feel the baby kicking. What could be sexier than that? Trust him when he says that you are sexy. Just sexy he can see in a picture but the sexiness he feels for you can only come from you and this far outstrips any nudie pic.