To start, I was pretty sure this was a troll post. I'm still undecided because this has to be one of the most messed up fact patterns I've ever seen. I cannot imagine how you've gone without sex for eighteen years. It would have had to have happened accidently at some point given that length of time. I don't mean this as mocking if this is real.
Assuming it is real... Dude. No.
You cannot stay married to this person. I don't say that as a challenge like "I bet you can't do it." I mean that you should have divorced about sixteen years ago... At the latest.
Was there some medical condition she replied on? Sex therapy? Did it ever come up in conversation? Were there ever fights about an eighteen years dead bedroom? I cannot imagine. I'm assuming that you at least brought it up in conversation a few times a year.
Here is the thing. It looks like you're successful in your own career. You're getting in shape. There are a few decades left. You've been warming the bench in your own life for some of the best years while your wife has been carrying a torch for a married man secretly.
Again, I still can't imagine this situation is real. The only possibilities left are that you're a combination of trauma bonded and scared to rock the boat to the point of absurdity.
So, let's try a little thought experiment... Put yourself in your wife's shoes. How does she have to look at you to stay married? Why? Why stay married to you? There's finances and appearances. That's it. There can't be anything else. There's no passion. There's no desire. There's no intrigue. There's nothing that would define love in any definition. To her, you're a nice guy walking wallet that's meant to be maintained and controlled. That's it.
My thought is this. You're going to divorce. There's no way around this. She's robbed you of years upon years of your life. She doesn't love you. She doesn't care about you. She is lost in a world where you're there for the hard stuff, but she doesn't really like you "that way."
My thought would also be this. How old are your kids? If they're older, get out of the house, retain a lawyer, change your phone number and don't be in the same room with her ever again without a recording device. If they're younger, find out how to get out of the house. This woman is exactly the type to destroy your life once you light a match. Gather your evidence and bolt out the door. Otherwise, bad stuff could happen.
You said she sat on your lap and she kissed you... Dude. I'm sure it feels good to have her want you after a lifetime of spurning your advances, but turn that around. She's been physically repulsed by you for two decades. Think that turns off in an instant? What thought process would she have to go through to get close to you in the hopes you won't leave? "Well, let's put on the lipstick, suck it up and see if I can smooth this over using my old tricks. I wonder if he wears boxers or briefs... I forget." Have you ever been with someone that the next morning you woke up and thought...well that was a mistake and I feel disgusted with myself? That's how she feels before she touches you. Her mind has been wrapped around another person for decades to the point where she doesn't want to be with you.
This does not turn off right away. It's fake. And if this manipulation doesn't work, she will try another. And another. Until she has the prop, you, that she can continue to show to the world.
You need some distance man. Some real highway distance. But you need away from her and you need to do it quickly. There is nothing to save here.
I know. I went though the whole thing too. Mine was cheating most of the marriage and I lived with a mostly dead bedroom for the last four. I know where your mind is and I get it. And you need time to process this. But you can't be with her. You just can't. It's impossible unless you want to live a sexless existence with someone that doesn't love you in order to keep up appearances at holiday parties.
Get a lawyer, move out. Nothing good comes from dragging this out any longer.
There were people that told me the same things and I didn't want to listen. And I get it. It's nice to live in the fantasy you've constructed for yourself but it's an illusion. Maybe you like the illusion. And maybe the illusion us fun.
I can tell you that kissing someone that wants you is way, way better than the illusion.