Hello and welcome to the board. I am going to pipe in with something that has not yet been brought up-Don't you think your wife could be just as unstimulated and longing for more as you are?
I guarantee you she has a fantasy life that includes others just as you do. Sometimes when relationships fall into such infrequent expressions of physical love, it is exactly that that holds them back- they are both feeling unsatisfied, wanting more, having secret thoughts they don't think they could ever tell their partner, living internally, and after 19 years, I am sure it has become an "issue" and every time you do have sex, there are years of starvation hanging over both of your heads, which certainly doesn't garner intimacy.
Add to that you perhaps resenting the fact that you have to be grateful for this rare chance and she feeling resentful that it can't be natural and free and more satisfying too--- I am definitely speaking from my own experience and perspective here--- She may feel just as terrible about the "obligatory" aspect of it.
I say all of this because I have been sex-starved. I have gone a month and more without having sex with the man I love, even as I saw him trying to approach me again and again. A large part of my iciness I think was due to feeling trapped by the relationship (my own trap, of course) and thinking of all the love and sex with others that I was "not allowed" to have because I was married.
This was one of the main issues that led to us almost getting divorced in the spring- Unfortunately I waited until I had nothing to lose to bring up my feelings and needs, finally told him all my secret thoughts. I couldn't believe how receptive he was! We are now having beautiful, open, frequent sex and one of the things that has brought us together is discussing polyamory, although we have yet to bring anyone else physically into our relationship-( I am getting to know someone I met online)
So maybe it would do a world of good just to talk about your fantasies, your sexual needs with her- even if polyamory is not an option, I bet there's a wild, passionate side of your wife you have never seen because it wasn't safe to talk about your dreams of sexual abundance.
I wish you luck, and the lifting of burdens, and all the love in your life that you need.
"Rocks will open and make a way for the lover."
~Hazrat Inayat Khan
I love Catfish and Charlie.