There are some great answers in this forum! If the sex completely stopped happening, I'd have to question "why?". I'd re-evaluate the relationship and then make a determination...fight to save it, or end it!!! Cheating IS NOT an option in my book!!!

Truth : Untruth
I see a difference one to the other. Cheating is relative to Untruth as Faithful is relative to Truth.
I cheated. My dishonesty exposed my betrayal of our vows of faithfullness wrecking my marriage. Later, when all truth had finally been revealed and my treachery was fully known, I was no longer compelled to cheat. By then I could express my desire to have an affair with a man I knew.
My wife chafed, chagrined spoke out loud, "Do what you want to do." My man friend his wife and I, made love and enjoy each other's sexual energies.
My wife and I made love again and again throughout the times of my open extramarital bisexual affairs. But, my failure to control and seek sensual/sexual balance in my ravenous sexual appetite is the cause of my bringing destruction to my marriage. Had I known or been willing to get polyamory advice about honesty and openness, I might still be married and my sensuality/sexuality with other persons and couples outside my marriage would continue openly with full approval of my wife.
She loved me. But, I had already broken the faith, betrayed her trust in me. She stoodfast. Betrayed, she wanted me to choose. Her, or more sex. We eventually divorced.

My marriage had deteriorated to the point of zero sex, zero intimacy, not even "hallway sex" (you know, passing each other in the hallway and saying 'fuck you'). After being married more than 20 years, after almost 5 years with no sex or intimacy, I began to cheat. Several years and several affairs later, I finally left the marriage. Why did I take so long? A combination of things, but mostly I don't think former-hubby cared about what I was doing, it was easier to stay than to leave, and I hadn't yet met anyone who inspired me to want to deal with divorce and the resulting fallout.

I'm not proud of myself for having done the things I did, but I do have a very different perspective as a result. I now have zero tolerance for cheating (within my own relationship), and, having formerly been a world-class liar, I won't buy any excuses or bullshit stories, as I've probably used them myself. Burn me once and we're done. Period.

Maybe I'm a hardass about it because I've been there and did it sooo well. Nope, I'm not proud of the things I used to do, but I have come clean with family and God and I'm not going to that place ever again. If my relationship ends up sexless as well as intimacy-less, and I can't live with it, I'll end the relationship before starting something new. I would owe that to him as well as to myself.

I have cheated in the past. And, honestly, it was because the person I love the most (who I am with now) and I split up because of family differences and I started dating another guy. The guy I'm with now came back around and we started talking on the phone and the next thing I know...the boyfriend takes a trip and I went out with my "ex" and could not keep my hands off him. I was so happy to see him.

It's not something I am proud of. But as someone else before me said, cheating is a statement that comes directly from the heart, not the body. I didn't cheat because I was horny, I cheated because I didn't love the guy I was dating, at least not as much as I am with the "ex" whose child I am currently carrying (not from that incident of course, as that was long ago lol).

They say once a cheater, always a cheater but that is not true. Would I ever cheat on my guy now? Hell no, I would not.