Understandable? Yes. Unreasonable? Also yes.
First of all, if they're getting a divorce, he kind of has to talk to her. If they're staying in contact, I'm guessing the divorce is somewhat amicable, but it's still a lengthy, drawn out process even when both sides aren't trying to use the courts to crush each other. Papers have to be signed, division of property has to be chosen, custody of children has to be hashed out...even the friendliest divorce is rough.
Beyond that, it's reasonable to be uncomfortable that he's still talking to his ex, but front and center...they're getting divorced. Generally, people who get along well in a relationship don't get divorced. That's not usually how it works.
Definitely you need to talk to him about it, and he needs to respect it makes you uncomfortable and talk about what it takes for you to be comfortable. But meet him halfway and give him a little trust. If you've been dating for seven months, he wants to be with you, not her, and it's really unlikely divorce proceedings are going to change that. Demanding he cut off his ex is only going to make him start talking to her behind your back...and that's just going to make things worse.
Ha!! My brother is going through the same situation with his soon to be ex wife and new GF. The reason why he talks to his wife is because they have children together and since they are both getting joint custody he has to talk to her on a regular basis. His new GF tried to make him choose and he told her that his kids come 1st no matter what. He told her that she knew his situation when they met so if she is not OK with that then she should hit the road.
She stayed with him but on occasion she still brings up the fact that he has contact with her.