You have been given some pretty sound analysis - there is something else going on besides, "not in the mood."
Choices are (as some of the others suggested):
1. Confront her them straight out about the fact they are having sex, which kind of precludes' "not in the mood," unless as someone suggested, hubby is pressuring her.
2. Confront the situation in a different way, by telling them that your needs are not being met in the relationship. List them and ask if they are going to be willing to meet those needs.
3. Confront the situation from yet another approach and state that since they are not meeting your needs during her "not in the mood" dry spell, you will began looking for another partner to address them.
I was with a couple once, and everything was fine - no issues with sex - until the wife realized the husband and I had grown to love each other. Then all bets were off. She went 180 degrees. (Wasn't love kind of the point?)