Tosafos (d.h. l'beischa) writes a very interesting yesod. Tosafos asks, how can the pasuk of "l'beischa" come to teach that a husband can bring bikurim (and read the parsha) on nichsei m'lug produce, since the husbands rights to fruits of nichsei m'lug is only d'rabonon, so the Torah cannot be making a reference to that? Tosafos answers that sometimes the Torah bases itsefl on assumptions that were normally done, albeit not required until the rabbonon came along and implemented it as a requirement. Even though a husband is only entitled to fruits of nichsei m'lug m'drabonon, it was the norm for a wife to give the peiros to her husband, so the Torah makes the assumption as if he owns the fruits of her nichsei m'lug. Similarly the gemara in Kiddushin 4a implies that the Torah gives the father ma'aseh yadayim of his daughter in exchange for feeding her, even though m'doraysa he is not required to feed her, since the norm was to feed her.
Similarly, Rashi in Parshas Chayei Sara (25:6) writes that the difference between a wife and a pilegesh is that a wife has a kesuba and a pilegesh doesn't. Ramban asks that this can't be the Torah difference between a wife and pilegesh, since kesuba was a rabbinic institution and not d'oraysa? Ramban answers for rashi:
אבל אפשר שגם בני נח כאשר ישאו להם נשים כמשפטן בבעילה היו נוהגים לכתוב להן מהר ומתן, ואשר רצונה שתהיה להם פילגש וישלח אותה כאשר ירצה ולא יהיו בניה בנוחלים את שלו, לא היה כותב לה כלום