ForGump - I might have a similar song to sing here. My own relationship had the same sort of thing.
I recognized fairly early on that my W used sex as a power and control thing and "got off" on being able to manipulate me by it. I recall seeing the look of triumph in her eyes after a particularly good session. Shortly after we were engaged our sex life also took a nose-dive. There were bumps up from time to time when she wanted something such as the house I am living in or our second child but otherwise it was "the duty". Since I loved her so very much I accepted this. Does my W actually have a strong sex drive? I don't know. Does your's? I know even less. Or is it in fact another drive that has an outlet via sex.
Analysis of this might be interesting but is it actually meaningful? Sorry - going through a bit of a rough patch at the moment myself.
On BD H52, W50 T27, M26 S21, D23 BD-9-Mar-16 D-15-Jan-18 Final-19-Apr-18 I am a storyteller. The story may do you no good. But a story is never for the listener. It is always for the one who tells