I had one underlying reason in my prior marriage - "My choice. My responsiblity" (Otherwise known as "you made your bed, now lie in it"). That reason lasted 10 years then I recognized that my responsibility extended futher than just that one choice - it extended to my children and myself.
In my current sitch it comes down to the overall decision that sex is not a reason to end a good, working partnership with someone I respect and love. Underlying that are probably some icky personal issues about what I can reasonably "expect" out of life which is probably something less than I might actually want - I am good at self denial. In this R it just happens to be in the area of sex whereas in another it might be another issue - money, parenting support whatever... For whatever reason, God put me here in this marriage, at this time to work this one out. Leaving for this reason instead of working through it just isn't an option. I'm also not too good with the idea of "failing" again.