Why is cheating so bad?

Well let's just say you adore this person, and they adore you. You spend years together, enjoying each others companionship. Then one day you realise that they have been with someone else.

You are hit with a lot all at once. You were lied to, played the fool, they clearly weren't happy, your special bond wasn't so special after all, you no longer trust them, you see a nasty side of them you didn't realise was there, you feel betrayed, you start to question yourself and why they cheated, you feel like the other person they cheated with is more appealing than you, so you lose confidence, you get mental images of all the things they do together, you feel alone, then you get angry. You no longer trust that person, and have issues trusting others...

It's a long list that goes on and on. It just feels shitty to have someone you cared about so much, toss everything aside because they feel they can do better. 

once you get in a relationship you'll understand. You make a commitment to that person, basically a promise. Saying "your mine, and nobody else's." for however long you date. and once guy/girl cheats you break the promise and it hurts a lot. Feels like your not good enough for them. It breaks your heart, that's why it's bad.

No....there is no need for list of do's and DONOT in a relationship. Who wants a robot? Only a narcissist would. You know what's right and wrong. If your in a relationship with someone that has cheated it's your choice to forgive or let go.

Cheating or betrayal generally is one of the reasons there's so little real love in the world.  Even long drawn out marriages are toxic and fake.