Here's the thing that she may not understand, and it took me many many years and conversations and failed relationships to understand.
Men and sex is far, far, FAR more complicated than talked or joked about. I, like so many other women hear the jokes about men think about sex every 30 seconds or that 1 out ever 5th thought is about sex and yada yada yada. It is so much more.
Counseling is a must for her specially. She needs to understand that being intimate with you is more than about you "getting off". I have come to understand that, not only is it healthy physically for you, but it also makes you feel more confident, desired, appreciated, and loved. On top of that, it is a HUGE stress reliever for not only you men but for everybody. She needs to hear from somebody other than you, that sex is important to you in more ways than one.
You're not all that much different as us in seeing sex as a way to connect on a much higher level. Constantly denying you your needs builds up and will eventually be too much for you and whether or not you want an affair, it may inadvertantly come to that. You'd have to make a choice of being unsatisfied or being unfaithful and that is not fair to you. Yes if you cheat that is all on you, but it would be you're unhappiness, and being unsatisfied that led you there.
Go seek professional help, if she doesnt want to go, go on your own and let her know you are doing this. Let her know why you are doing this. It will hurt her to know you are unhappy with how things are, but doing this together should show her that you still love her. You're willing to seek professional help so it doesn't come to affairs. You desire her and want to be with her, and it only makes sense that she should feel the same, and show you that. Her sex drive may not be at tour level, or even where it once was, but she should be willing and wanting to make you happy, and sex is a sure fire way of doing this. It isnt about womanly or wife duties as much as being there for the one you love no matter what.
This medical issue isnt something new, and I'm sure there are treatments and plans out there to help her with it. I havent ever heard of it, but it cant be so uncommon as to not have treatments. She may feel embarrassed about seeking help, but again, for the sake of a healthy sex life, she should be willing to put her feelings aside. Be there for her evey step of the way, and let her know you're supporting her. It wont make things that much easier, but she will knownshes never alone in this.
You love her and are willing to do what it takes to be happy with her, then it is only right, fair, and equal, that she do the same. If she isnt wanting or willing to work things through... I hope it does not come to that.
I wish you the best love