Chapter 5 addresses sexual fidelity. It is hard to pick apart what is my modern lens from what is in the text sometimes, but it seems to me that author exhorts his son to stay faithful to his wife. To not share his wife with other (I'm looking at you Abraham!), and to not have sex with either the wives of other men or wayward women. And the reason, the theme of the book, is to do otherwise is to live less well.
Adultery is also the concern of the second have of Chapter 6, which includes "But a man who commits adultery has no sense; whoever does so destroys himself." Proverbs 6:32. According to the headings, this theme continues on to Chapter 7. That's a lot of anti-adultery.
NB.: something else that Jesus wasn't the first one to say, "Do not desire her beauty in your heart,
and do not let her capture you with her eyelashes." 6:24.
But it's not all sex. Let's end with some more general more morality, from Chapter 6.
16 There are six things that the Lord hates,
seven that are an abomination to him:
17 haughty eyes, a lying tongue,
and hands that shed innocent blood,
18 a heart that devises wicked plans,
feet that hurry to run to evil,
19 a lying witness who testifies falsely,
and one who sows discord in a family.