I am late in replying to this, but here are my findings with regards to fornication: the Greek word for fornication is "porneia", and it includes any kind of immoral sexuality. Now this is a broad term but unfortunately it I used too broadly today. It does not include "sex before marriage", there is actually no law against this is the Bible. It does include adultery, harlotry, some forms of incest (we know certain forms were permitted), and idolatry. Now, harlotry is interesting as there are various words relating to this word in the Greek and Hebrew. The two main words I've usually seen are "qedeshah" and "zanah". From what I've gathered, the former means "temple devotee" and the latter simply a "harlot/prostitute". And from my understanding, generally when Paul and others speak against fornication, they are speaking against "qedeshah" or temple prostitutes. Why? Because sexual acts with these temple devotees were a form of worship to other "gods"; it was idolatry. Now, when I've seen "zanah" used in the Old Testament, it's not always negative. For example, a non-negative use is in Judges 16:1 where Sampson sleeps with a "zanah". I say non-negative because we do not see condemnation for this act. Whether it is positive or not is up to one's own view of regular prostitution. So, coming back to concubinage being fornication; it cannot be. Fornication is either idolatry or theft within the sexual sphere.
I would like to end by stating that these are my findings. I always am open to correction if someone has found new information regarding the hermeneutics of Biblical texts. Thus, please do your own research, don't take my word for it.
With peace and love