We read in
Mishneh Torah-Hilchot Ishut 1:4
“Before the Torah was given, when a man would meet a woman in the marketplace, and he and she desired, he could give her payment, engage in relations with her wherever they desired, and then depart. Such a woman is referred to as a kedeshah. When the Torah was given, (relations with) a kedeshah became forbidden, as (Devarim 23:18 states): "There shall not be a kedeshah among the children of Israel." Therefore, a person who has relations with a woman “leshem zenut”, without kiddushin, receives lashes as prescribed by the Torah, because he had relations with a kedeshah”.
The expression "leshem zenut" used here by Rambam is usually translated into English as "for the sake of lust" or "for the purpose of fornication"; however, it is not clear whether the text should be read "leshem zenut, that is without kiddushin", or "leshem zenut and without kiddushin". The difference between the two hypotheses of interpretation is very large, since in the first case the textual incision "leshem zenut" would be a mere explanatory note relating to the absence of kiddushin; in the second hypothesis, however, it should be concluded that,according to Rambam,the commission of such violation requires the coexistence of two different conditions: the absence of kiddushin and the presence of "leshem zenut".
Ramban's interpretation is oriented in this second direction, as explained in
Teshuvot haRashba Meyuchas LehaRamban No. 284
“And also in regard to the words of the Rambam (Rabbi Moshe ben Maimon), of Blessed Memory, they are not intended to forbid the Pilegesh relationship to a commoner and permit it to a king. Rather, this is what he said: “a person who has relations with a woman “leshem zenut”, without kiddushin, receives lashes as prescribed by the Torah, because he had relations with a kedeshah”. “Leshem zenut” means he contacted and engaged in sex with her and did not render the relationship exclusive to him for the purpose of establishing a Pilegesh form of relationship, which thus rendered her a kedeshah.Therefore, Rabbi Moshe of Blessed Memory did not say that anyone who has sex (simply) without kiddushin is flogged”.
How shared is this interpretation of Ramban in the Jewish tradition?