Zeno in Fragment 244 wrote against adultery:
ἐκκλίνουσι τὸ μοιχεύειν οἱ τὰ τοῦ Κιτιέως Ζήνωνος φιλοσοφοῦντες … διὰ τὸ κοινωνικόν· καὶ <γὰρ> παρὰ φύσιν εἶναι τῷ λογικῷ ζώῳ νοθεύειν τὴν ὑπὸ τῶν νόμων ἑτέρῳ προκαταληφθεῖσαν γυναῖκα καὶ φθείρειν τὸν ἄλλου ἀνθρώπου οἶκον.
"Those who pursue the philosophy of Zeno of Citeus avoid engaging in adultery on account of ideas that are commonly held. For [they say that] it is even against nature for the creature of reason to engage in adulterating a woman who is still legally married, and to engage in ruining the home of another man."
(For the record, Dr. Jim Voelz helped me revise the translation -- although the above translation is "looser" than Jim would render it.)
In any case, came across this video of Professor Charles Rice from Notre Dame. He is a law professor and in the video below he provides a brief overview of natural law. Other than his opening prayer to the Regina Coeli, I didn't have much to disagree about in his presentation. He helpfully described what natural law is and the shift that has happened since the Enlightenment -- relativism and individualism. It is worth the 9 minutes or so he takes to explain it.