It was adultery, pure and simple. It happened, but not as an example for us to follow. Sin mentioned in Scripture is never mentioned for the justification of immoral behavior.
Hagar could not received the marriage of Sarah and Abraham they were not of the same faith so they marriage was until death we part.
Just like most marriages today are until death do we part so that could be called a bondman marriage!
It is not that they were living in sin it just that under those conditions Hagar could not receive an increase after life.
Yet somewhere along the line the Lord did give Ishmael and his people a covenant!