This sounds like something hormonal to me. Men and women are meant to sexually balance each other out. This part of nature keeps our sexual activity "healthy". Here's how it generally works with healthy adults:
Men -
The more they get, the less they want.
The less they get, the more they want.
Women -
The more they get, the more they want.
The less they get, the less they want.
These factors, when put together, help balance each other out so that a healthy "medium" is maintained. As we all know, too much of a good thing is still a bad thing.
So, if you have gone through lots of sexual activity, and you don't find your drive increasing, then you might want to look at what else is going on. Look at your environment. Is your house clean or is it a mess? The first thing you can tell about a person's emotional state is to look at their environment. Your home will often reflect how you truly feel. That's not always 100% of course, but if you are depressed, likely, you can't seem to find time to clean or be orderly because you're napping a lot, zoning out a lot, binge eating... etc...
On that subject, also look at your physical health. Diet and proper exercise are very important to a strong libido. Are you eating healthy? Are you getting at least 20 minuts of elevated heart rate exercise per day? Have you had a checkup with the doc lately? If not, get in to see one right away. Just like men can develop sexual disfunction because of health, so can women. Especially moreso because of hormonal imbalances that often occur before, during, and after menopause. Make sure you tell your doc that you are having issues with interest in sex and ask if he/she believes it could be due to some physical issue. For an emotional issue, it never hurts to go get an evaluation from a professional. It doesn't mean that you're psycho or that something is "wrong" with you, but these professionals exist to help people and they're often very good at what they do.
Whatever the case, you should take action. Your partner should not be suggesting getting sex elsewhere, especially openly, unless he is desperate for satisfaction.
Also, I am not a doctor, just an old "**** and vinegar" army vet. So, you can't take my opinion as "informed". I'm just guessing.
