Sex in the Five books of Moses is not symmetrical. A woman can be married to only one man. A man can be married to many women.  ניאוף  adultery is when a man has sex with a woman who is married to another man. That gets the death penalty. It is in two lists in Leviticus  that go though the עריות.
Sex outside of marriage is  not ניאוף-adultery. It comes under the category of פילגש concubinage.

עריות in Leviticus are generally close family relations. But adultery gets thrown in also in both lists.

There were a good number of people in the Old Testament that had concubines. One well known such person was the friend of Joshua the disciple of Moses. [כלב בן יפונה ] He was not considered a sinner because he has a few wives and few concubines. In the Five Books of Moses he is praised with a kind of praise that is not applied to anyone  else in the Torah, וימלא אחרי השם "He was filled with God."

I am not saying this is the best option. And we do have the Rambam who says a concubine is forbidden to anyone but a king. All I am saying is that there are plenty of opinions to depend on here that allow a girl friend.
Still if you can get married in the traditional way that is best. But for some people it is hard to find such a  situation.