Is revenge sex fair after being with a cheating partner?

It isn't a healthy reaction. Either forgive or move on, but getting even isn't mature or healthy and won't help anything.

If you cheat on your SO then you're not any better than they are. 

The cheating SO does not deserve that form of energy from you. Honestly I'd channel that energy on doing positive things that make you feel better.Yes you are probably hurt and angry, but when we feel this way we tend to act wrecklessly and irrational. Don't forget that there are consequences attached to sex like sti, pregnancy, and more emotional disappointment. This guy screwed up, he straight up screwed up. He's the fuck up, not you. I don't care what the Internet tells you or what your hurt feelings are telling you HE made that choice, HE decided to go forward with it. HE could have been an adult and worked out issues with you, or left you the right way with complete honesty. HE didn't, and that tells us more about HIS character than yours.

Don't do it

Leave him. you're the one you're really going to hurt in the end! It will feel good in the moment but you will lose respect for yourself. Never worth it! 

Just be the better person...you're not going to feel as satisfied as you think. It's a lot more satisfying being the better person and being able to throw that in their face.

It's not the right  thing to do. I did it once and it didn't make me feel any better about myself or the situation. Just leave the cheater and then you can sleep with whoever you want.

Someone once told me... 2 wrong don't make 1 right. Probably at the moment you make the choice to "cheat as revenge" is a choice you make when you are angry. But then, when things calm down you will feel like crap.. May be not because of what you've done but because of the emptiness you'll feel inside you.. But this is just an opinion. Do what's best for you.. Sorry to hear/read what's happening :/