Why is it so hard for a man to be understanding and compassionate towards his pregnant partner?

There is no greater test of the strength of a relationship than having a child together. It changes you, it changes your partner, and it changes the relationship. Some relationships can handle it some can't. I strongly suggest coupled counseling. It is a great way to get feelings know without damaging the relationship and a great tool to learn how to communicate and cope. Your sex life you described is rape by coercion. It doesn't matter if you are married or no a woman has a right to say no and the partner MUST grant that. It's a law in the U.S. he could be arrested for spousal rape. If the touching is unwanted and he KNOWS it's unwanted but does it anyway it is molestation. You need therapy. If he won't go to couples therapy then try to go on your own. I'm not being judgmental I am being honest.