Personally I can not imagine having sex with only the same person for 20 years...
I have had sex with the same woman for almost 30 years, and she's the only woman. But now I lost interest in sex already, I guess she too. Nevertheless, she's always "available" whenever I want it, which is getting fewer and far between. No plan to see doctor, no need for Viagra, we are cool with it, no interest means no erection, so no problem.
