I don't feel that sex is the defining characteristic of a wonderful relationship. I can easily see where two people in a long standing relationship could drift sexually apart because of their different needs/desires. I don't think this has any intrinsic meaning other than what you guys assign to it.
There's a lot of ways to fulfill the intimacy quotient that doesn't involve conventional sex/orgasm etc. Cuddling and closeness is always available.
On the other hand, there may be some "stuff" going on in the background that's not being talked about. That can certainly kill the sex drive. Trust issues ? Insecurities ?
You can choose to 'dig' for these things, or just accept the position you are in for a time and see what happens. Time exposes/solves many things.
You say you are both 'fulfilled' from other directions so the frustration element should be minimized. That always helps.
I say look at it as a curiosity and don't make a huge deal of it right now. See what happens over some time.