I suspect this will be the most responded to of all reasons, especially for the men!

After being married for more than 7 years (itch, itch), the sex life was pretty stale at home, when it was existant at all.  I began to keep count at one point and was disappointed to learn we were down to one event every two weeks, maximum.

There are probably "better" men out there that could suck it up and deal with that.  Another subset also that might have strained the marriage by pointing this depressing statistic to the wife.  (I suspect it would have made her skip another week if I did that).

For me, it was the reason I began looking for something on the side.

For those that know our story here, Jane and I are way more than just F-buddies.  But as a typical guy with a pretty typical story, that's how it began for me.

I suspect that many affairs evolve from whatever triggered them to begin in the first place, so for those that respond to agree that sex was the driver, let us know if that is still the case, or whether you gained a great friend as well as a lover.

I certainly did.

My wife and I still have a decent sex life.

For me, I just needed some variety.  It grew to be much more than just sex, and is now a very important part of my life.  But that's how it began...just needing something different.