The bitter waters only work if the husband is free from sin TB Sotah 28

 I don’t think that the rabbis ever really wanted to employ the sotah’s bitter waters test to determine whether the wife was innocent or guilty of committing adultery. (Perhaps down deep in their heart, they were skeptical of the efficacy of this miraculous test.) They kept on adding conditions which but prevent this test from happening. We learned on the previous daf TB Sotah 27a when either the husband or wife were mute, lame, or an amputee, she would not drink the bitter waters.

Today’s daf Sotah 28 adds one more important condition. For the bitter waters to work, both the wife and husband have to be free of sin. “(We learn in a baraita -gg) “And the man shall be clear from iniquity, and that woman shall bear her iniquity” (Numbers 5:31), indicates that only when the man is clear of iniquity does the water evaluate the fidelity of his wife, but if the man is not clear of iniquity the water does not evaluate the fidelity of his wife” (Sefaria.org translation)

There is a disagreement what this sin is. Once the wife is warned and then secluded, she is a sotah and is forbidden to have relations with her husband until she is proven innocent by the bitter waters. Rashi says the sin is the husband has intercourse with his wife while she was a sotah.

Moses Maimonides expands the definition of the sin to include any forbidden intercourse with his wife. “Whenever a man has engaged in forbidden relations from the time he attained majority onward, the curse-bearing waters do not test [the fidelity of] his wife. Even if he engaged in relations with the woman he consecrated while she was living in her father's house - which is a Rabbinic prohibition - the waters do not test [the fidelity of] his wife. [This is derived from Numbers 5:31, which] states: ‘The man will then be free of sin, and the woman will bear [the burden of] her sin." [Implied is that] when the man is "free of sin," "the woman will bear [the burden of] her sin.’” (Sefaria.org translation, Mishneh Torah, Sefer Nashim, Hilkhot Sotah, chapter 2, halakha 8)

Ramban expands the definition of sin even further to include all the children of the husband’s household in his commentary on our verse. If they are not free of sin, the bitter waters do not work.

By the way, the rabbis say hold what is good for the gander is good for the goose. If the wife is guilty of adultery and the bitter waters effectively kill her, they will have the same effect on the paramour even though he doesn’t drink the bitter waters. “MISHNA: Just as the water evaluates her fidelity, so too, the water evaluates his, i.e., her alleged paramour’s, involvement in the sin, as it is stated: “And the water that causes the curse shall enter into her” (Numbers 5:24), and it is stated again: “And the water that causes the curse shall enter into her and become bitter” (Numbers 5:27). It is derived from the double mention of the phrase “and…shall enter” that both the woman and her paramour are evaluated by the water.”  (TB Sotah 27a, Sefaria.org translation)