I wouldn't tolerate it, personally. However, if a married or attached girl wants to bone, I have no personal objections if I'm single.
For most people this is the definition: adultery [ əˈdəlt(ə)rē ] NOUN noun: adultery · plural noun: adulteries voluntary sexual intercourse between a married person and a person who is not his or her spouse: "she was committing adultery with a much younger man"
I think to be an adulterer one has to have definite sociopathic leanings. It is a deliberate betrayal of epic proportions.
There are levels. For instance a drunken hook up on a business trip is not that same as doing his/her best friend on the regular.
Same victim, same betrayal, same level. You don't get points just because her best friend is repulsed by you and you can only get hookers. You could murder a stranger or murder your wife's best friend. You're still a murderer.