Ibn Ezra on Numbers 5:31

תשא את עונה. אם נטמאה: SHALL BEAR HER INIQUITY. If she was defiled.

וטעם ונקה האיש מעון. כי אם עבר עליו רוח קנאה הנה היא אסורה לו שישכב עמה והשוכב יש לו עון. יש אומרים שטעם הסמך פרשת נזיר לפרשת סוטה שיהיה לה בן נזיר אם לא נטמאה ולפי דעתי כי נסמכה בעבור נזירת האשה שהיא הפך המועלת כי רובי העבירות סבתם היין ואשה שלא תתקן שער ראשה איננה מבקשת להבעל: Our verse states, And the man shall be clear from iniquity.⁷¹If the woman is exonerated and he has relations with her. For if a spirit of jealousy came upon him she is prohibited to him. He is not allowed to be with her.⁷²If a man suspects his wife of adultery, he is prohibited from having sexual relations with her until she is exonerated. If he sleeps with her, he is guilty of sin. Some say that the section dealing with the Nazirite follows the section dealing with the wife who is accused of adultery because if the latter was not defiled, then she will have a son who is a Nazirite. However, I believe that the chapter dealing with the Nazirite follows [the chapter dealing with the wife who is accused of adultery] because it lays down the law regarding a female Nazirite who is the reverse of the unfaithful wife. Now wine⁷³Which the Nazirite forswears. causes most sins. Furthermore, a woman who does not fix her hair⁷⁴The Nazirite is prohibited from cutting hair. does not seek sexual relations.⁷⁵Thus the female Nazirite who abstains from wine and does not cut her hair is the reverse of the unfaithful wife who drinks wine and does cut her hair.