Adultry is the act of breaking a vow you have made with God. it doesnt necessarily have to be that you had sex whith another when you are married. You could vow to God to stop eating meat, have a pork chop, and you are committing adultry.
Anyone who is a Christian, who is divorced and re-married is lving in adultry according to the Bible. Moses allowed re-marriage after divorce, if the divorce was for marital unfaithfulness. But in the new testament, Jesus put it point blank. If you remarry after divorce for any reason, it is ADULTRY.
This makes sense when you consider the populations that Moses and Jesus are dealing with. To block all re-marriage in Moses' day could have meant the demise of the entire tribe.
But by Jesus' time, the population was enough that it was not a threat.
When you marry in a Christian ceremony, you not only make a vow with the person you marry, but you also make a vow with God. It is O.K. in God's eyes to break the vow you made with the other person, but not the vow you made with him.
Adultry is often committed by those who dont believe as well, but to them it's no big deal.
And this is why it is widely tolerated today.
Almost all organized religeon churches today ignore what Christ said about re-marriage after divorce. Very sad.