Source: J.J. Collins, "Testaments," Jewish Writings of the Second Temple Period, ed. Michael E. Stone (Philadelphia: Fortress, 1984) 334.
"The exposition of a particular virtue may be illustrated from Test. Reuben. First, the patriarch declares his theme: 'Pay no heed to the face of a woman...' (3:10). He then provides as an illustration his misdemeanor with Bilhah (3:11-15) and concludes with an exhortation to 'pay no heed to the beauty of women...' (4:1-5). This exhortation is complete in itself but is supplemented in 4:6-6:5 with another, similarly structured sermon. Again, the patriarch begins by enunciating his theme: 'For a pit unto the soul is the sin of fornication.' This time the example adduced is that of Joseph: 'how he guarded himself from a woman and purged his thoughts from all fornication' (4:8). The evil of women is further illustrated by the story of the Fall of the Watchers before the Flood (5:1-7). Finally, the exhortation concludes 'beware therefore of fornication' (6:1)."
The Testaments of the Patriarchs are dated to the second century B.C.E., but scholars believe it contains later Christian additions.
Most of the time, I've heard that Reuben slept with Bilhah to gain his father's power. That's one way people gained power in biblical times: they slept with their father's concubine. Solomon's brother, Adonijah, did the same thing, for he tried to get David's hot concubine, Abishag (I Kings 2). But the Testament of Reuben treats Reuben's action as a matter of lust: Reuben thought Bilhah looked good, so he took her, thereby defiling his father's bed (Leviticus 11; I Chronicles 5:1; Amos 2:7). Had the custom of taking power in that way faded by that time, explaining why it doesn't appear in the Testament of Reuben?
The passage is against lust, like some Jewish writings, as well as the Sermon on the Mount in Matthew. And the hatred of fornication leads to misogyny, which is what we see in Ben Sira. Fornication had bad results in those days. For instance, it could disrupt a family's inheritance. That's why men had to resist temptation, as hard as it may be.