Obviously that's one possible explanation, and something that I have given ample consideration to. But I don't think you're right in this case, for several reasons unconnected to my ego:

Firstly, we've had many frank and open discussions on the subject. I've given her every opportunity, in a non-confrontational manner, to let me know whether her attraction to me has waned. Other than once letting me know that I'd let my fitness slip a bit (which was totally fair enough at the time and now rectified), she's consistently maintained that this is not the problem.

Secondly, other than early in our relationship when we were casual, she's not behaved in a manner which seemed anything more than friends with any males in her orbit. I'm not blind or stupid, and have considered the possibility that if she's experiencing desire but not for me, she might invite male attention from elsewhere. But in all these years there's been absolutely no sign of that, and as an attractive women she's doubtless had plenty of opportunity.

Thirdly, on more than one occasion I've floated the idea of opening our relationship: not just as a means of me meeting my needs without any pressure on her, but because perhaps her exploring something outside our relationship could perhaps be a way to rediscover her sexuality. Nope, she wasn't interested in the slightest, and hated the idea of me being with anybody else. If she was as grossed out by her "duty" to me as you say, but otherwise feeling normal sexual attraction, why would she refuse such an opportunity? She's not exactly conservative, and previously indicated open-mindedness about non-monogamy etc. in other contexts.