Well, you do now.
Jai and I have been in a monogamous relationship for 6-1/2 years now and enjoy sex as much now as when we first met...maybe more.
To your point, most gay relationships seem to fall apart after 2 years or so for the very reasons you mentioned. If one or both of the people in the relationship isn't being fulfilled (sexually) it would only make sense that they would part ways.
I enjoyed 15 years of Butterfly life in Thailand to the max. Seriously, no regrets whatsoever...hundreds of boys (if not more)...loved the thrill of the chase...spontaneity...variety, etc., but there comes a time (at least for me there did) where I wanted something more.
All of my expat friends over here are unhitched butterfly's. Some seem content with their lifestyles, and some seem discontent and bored. I guess it's just a matter of each person finding what works best for them.