Look, you could argue back and forth, until the cows come home, about whether sex is "owed" in a relationship.
At the end of the day, the Op has only had sex 5 times since the start of 2015. The only people (in my mind), who would find this tolerable are either:
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People who don't fancy their Partners, but stay for financial security
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People who don't fancy their Partners and are getting sex elsewhere
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People who are A-sexual
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People with an extremely low libido
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People who are dead
Personally, I would be climbing the walls and could not imagine still being in this relationship, some 4 years later. Because I love sex. And I fancy my DH.
As for all the holier than thou PP, berating Op for having an EA. Ridiculous! Made me think of the Friends Episode, where people express surprise that Rachael as going on a date, and she replies "Yes, I had sex with Ross two years ago, I'm all set".
I doubt Op went onto Ashley Maddison or Illicit Encounters, and found herself a man to flirt with. In all likelihood, a man at work paid her some attention, and given that she hasn't had any, intimacy, attention, flattery or sex in years, she felt suddenly a little bit attractive and flattered. Fuck's sake, is that so hard to comprehend? Unless you are dead from the waist down, I would imagine that was a foregone conclusion. Her "D"H is bloody lucky she didn't take it any further. Instead, she is still trying to fix her marriage. She deserves a medal in my view.
Ugh, and please stop with the "no means no" tripe. It's not that simple, and you know it. You wouldn't trot out that line if your DH said "No, I'm not paying any bills this year. No means no. End of"
Marriage is a partnership, and unless you are both a-sexual when you take your vows (and have agreed there will be no sex), it's changing the goalposts enormously, to take sex off the table after the vows have been said. No amount of "no, means no" makes that acceptable. The person saying it just sounds like a massively selfish arsehole who couldn't give a shit about their partners emotional well being.
"Yes darling, I know we used to be at it like rabbits before we got married, but I've got the diamond ring now, I've got all of the financial trappings of marriage, if you divorce me, I'll take half your assets and pension....and I've decided I no longer want sex with you, no means no, so you can't push me on that....what are you upset for...no means no....boo fucking hoo, suck it up"