Blimey, some people on here are so one dimensional. Remember Op, there are plenty of people on MN that have zero libido and will be unable to empathise, and will just say "you can't be happy then".
Marriage shouldn't feel like a prison sentence. It is hard I think, to have to stand by the decision you made at 18 years old. For Life.
I think it's entirely normal, to feel how you are feeling. You've been with your DH for 20 years, he was most likely your first boyfriend and possibly the only man you've had sex with? Totally normal that you might now, after all this time, start to notice other men, especially as you hit a time in life where you may feel that your youth is slipping away (it's not btw, but I think everyone feels a bit like that, when they approach milestone birthdays).
I married my first BF, when I was 20. I'm pretty sure I would have stayed faithful, but I did sometimes wonder what another man would be like. Unfortunately my DH cheated many times, and when I was your age, I as a consequence went off the rails and had some affairs. The excitement was off the charts, at the beginning, with the thought of what was going to happen. BUT....the harsh reality is that :
- Men who sleep with married women are predators. They promise the earth and don't deliver. You end up feeling like a fool.
- The sex is shit. They are very selfish in bed.
- The men are mostly fucking jaded and end up nasty.
- The men are never only seeing you. You'll be one booty call of many.
- The marriage will end and your kids will be devastated.
All of the above, drove me to have a mini breakdown to be honest.
It all worked out okay for me in the end. I had loads of sex and got it out of my system, then I met my now DH who had had a plethora of women before me. We both settled down, and neither of us has any "itch" to scratch because we've got it out of our systems.
The thing is, even if you believe all I've said above, you might just go ahead anyway, because the desire to shag someone else is so strong. It's very hard, because neither path is easy for you.