Rashi and Tosefot disagree about whom Rava is talking TB Kidushin 51

Today’s daf Kiddushin 51 analyzes two halakhic principles. Rabba states the first one. “Any matters that cannot be accomplished sequentially cannot be accomplished even if one performs them simultaneously.” (Sefaria.org translation) the Mishna on daf TB Kidushin 50b provides an example of what Rabba is talking about. “In the case of one who betroths a woman and her daughter or a woman and her sister in one act of betrothal, by saying: You are both betrothed to me, neither of them is betrothed.” (Sefaria.org translation) Since a man cannot marry a woman and her daughter sequentially, he cannot marry them simultaneously. The Gemara successfully defends Rabba against all difficulties brought against him.

Rava states the second principle in a disagreement with Abaye. “With regard to betrothal that is not given to consummation (קִידּוּשִׁין שֶׁאֵין מְסוּרִין לְבִיאָה), Abaye says it is betrothal, since the prohibition against engaging in sexual intercourse does not affect the betrothal itself. Rava says: It is not betrothal. Rava says: The Sage bar Ahina explained to me that this halakha is derived from the verse: “When a man takes a woman and engages in sexual intercourse with her” (Deuteronomy 24:1), as it indicates that betrothal that is given to permitted consummation is betrothal, whereas betrothal that is not given to consummation is not betrothal. (קִידּוּשִׁין שֶׁאֵין מְסוּרִין לְבִיאָה – לָא הָווּ קִידּוּשִׁין)” (Sefaria.org translation)

Rashi quoting the Gemara that when a man can not engage in sexual intercourse with a woman is the definition of “betrothal that is not given to consummation.” Tosefot ד"ה קִידּוּשִׁין finds this definition difficult. When the forbidden marriage only creates a negative prohibition, the kiddushin takes effect and there is betrothal. An example of this would be the Kohen Hagadol, the High Priest, who marries a divorcee. The Torah forbids this marriage and the couple is forbidden to have relations, but since it only creates a negative prohibition, the kiddushin takes effect. Consequently, the couple needs a get, a bill of divorce. If this is the correct definition, this poses a great problem for Rava. He should hold that the kidushin does not take effect at all.

To solve this problem they explain the definition of “betrothal that is not given to consummation” thusly. Only when the marriage itself creates the prohibition is the basis of Rava’s principle. An example of this would be a man wants to betroth both a woman and her daughter. The punishment meted out by the Torah in this case would be koret. Since betrothal to both the woman and her daughter would be punishable by koret, Rava holds that neither are betrothed.