It's simple really.
You need and want sex. She doesn't (so much). It doesn't mean she doesn't love you and as you say, you are moving forward in other aspects of your lives.
It's just one of the many differences that partners find out about partners along the way.
Most people have a lot a fair bit of sex at the beginning of a relationship but as the years go on, the sex life dwindles. That's what happens in life and with relationships.
If necessary, you could do the obvious, lighten her load, make her feel loved, give her aphrodisiacs.... You may get lucky. If not, well you could be fighting a losing battle.
Anyway, I may be wrong but hey ho.
