A single woman has an affair with a married man. Are they both adulterer's or just the one who is married?

First I’d like to know your opinions about adultery in general.

Then, in context of one of the Ten Commandments…
“Thou shalt not commit adultery”...

A pretty vague rule incapable of answering my original question. Is it only adultery for those who cheat on their spouse? The mistress is single… has she committed adultery too?

Let’s say the mistress does have a boyfriend… Maybe even a fiance’. Is that adultery if she cheats on a boyfriend or fiance’?

A few thousand years passes and Jesus comes along in support of the Ten Commandments… but Jesus provides some details telling us that adultery is committed even if we lust after someone, married or not.

So what guidelines do you personally use for adultery. If you were the single mistress or the single john, have you committed adultery by sleeping with a married person?