The whole issue of having a girl friend or in the language of the Bible פילגש I have noticed that a lot of people are not aware of. This comes up mainly in Chronicles 2:46 with the friend of Joshua כלב בן יפונה. I have mentioned before that Calev Ben Yefuna was the only person in the Bible that it says the amazing phrase: "וימלא אחרי השם" ["He went totally after God"].
The basic outline of the subject you can easily see in the Rambam and Shulchan Aruch.
The Rambam it is known is against this, and allows a girl friend only to a king. The Gra pointed out that that can't work in the case of Calev Ben Yefuna. [who had many girl friends but was not a king.] But the Gra has a different explanation of the whole thing. He says a פלגש is with kidushin but without a ketubah. The חלקת מחוקק and בית שמואל point out that even to the Rambam, a girl friend is only an אסור עשה that is a prohibition that is derived from the lack of doing something--that is making kidushin. [I can not answer for how the Rambam might have explained כלב בן יפונה.]
That is to say it is not that same thing as זנות which is a לאו a straight forward prohibition.
In any case, on the side of permission, we have at least the Raavad and the Ramban, and some say the Rosh also. The reason is the Rosh only mentions the problem of she might be embarrassed to go to the mikveh. People like the Radvaz and later achronim went through the trouble to find how many rishonim allow it, and as far as I know there is no doubt that the majority of Rishonim allow it.
This really would not even need to be necessary to mention except for the fact that I have heard people confuse this with adultery,-- which it is clearly not. This opinion I imagine can only have originated with people that can't read Hebrew. Adultery is a totally different story. It is an act of sex with a married woman. It has nothing to do with a girl friend. Thus a man can have many wives, but a wife can not have two husbands since each act of sex would be an act of adultery which gets the death penalty as we see in Leviticus 20
