Sex outside of marriage is consider fornication. If she is still legally married, it is consider Adultery. Both are talked about in the Bible. Both are sins and there is no difference in a sin. A sin is a sin.
Now, I don't know many people now a days that are not having sex, LOL. Doesn't make it right with God, but we still do it. Does it mean you are in jeopardy of Hell? Depends where the heart is with God and how you try to stop doing these sins and ask for forgiveness. I think we all do bad things from time to time and no one is without sin. This is where a lot of religious fanatics seem to forget. They are first to cast stones. Then there are people who see that is being hypocritical. I agree with them, we should all take a deep look at ourselves first.