My fiancée (22) had graphic phone sex several times with a male friend (18) while I was away and she was mad at me. Is this cheating, and if so, how bad?

Im no expert, and I think this question really depends on your feelings on the subject.

My opinion:

I would consider it cheating if I were in your situation.
In addition, I don't think there really are severities of cheating, she either did or she didn't...

Again this is only my opinion, I'm sure someone can provide you with a better answer.

This behavior is cheating and very inappropriate for couples to go outside the relationship. Not a healthy way to care for someone, mad or not.

It really depends how you consider it.

I'd consider it cheating. She might very likely do it again... even though you're engaged.

Its definitely cheating, because cheating is basically anything you do with someone else for some form of sexual gratification other than your spouse.

When you have any type of sexual feelings or sexual intmacy with someone other than your significant other it is cheating... becuase you are feeling something sexual for someone else.

My fiancee (22) had graphic phone sex several times with a male friend (18) while I was away and she was mad at me. Is this cheating, and if so, how bad?
I would say it was cheating to what extent I can't tell you. She is discussing sexual desires with another guy instead of you and it doesn't matter if she was mad at you or not, its still not right.

What I would be asking is, has it gone any further than just over the phone.

This is nasty! Run a mile and don't look back or communicate with your fiancé. Explain this is unacceptable behavior and you consider it as cheating. It is totally inappropriate for a healthy loving relationship.

Her behaviour is an absolute betrayal of your feelings and respect.

She obviously has little regard for you as your fiancé.
Will she give away "favors" to help "cheer up" lonely friends? If she's really mad at you, will she physically cheat on you as "revenge"?

Unless you have some kind of "open relationship" going on, I would seriously re-consider a future with this person.