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.