Again - kindly find the passage in the Bible that lists where concubinage is forbidden. You won’t. In fact, you will find many of the figures in the Bible had concubines and ‘slept around’ on their wives. God didn’t seem to have a problem with that.
I’d also point out that the original term (after the Aramaic) was the Greek word ‘porneia,’ of which “fornication” was the English word the King James translators chose. Unfortunately, as with more than a few translations they chose, it’s not really strictly correct. That link someone posted earlier actually has an interesting discussion of it: http://thenarrowpath.com/ta_divorce.php
Porneia translates ‘better’ as ‘sexual immorality’, but even that doesn’t really grasp the concept - which is a whole other discussion...
And yes - pretty sure I’m a Christian, even though we apparently differ quite radically on interpretation of Bible verse. Here’s Matthew 15:19:
19 For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies:
Of note is that Christ specifically separates out adultery from fornication. They’re two different things - more to the point, of that list, two are not specifically listed as sins under the Mosaic law. He’s saying these are bad ideas (and they very much are) but not everything on that list is prohibited.