Kiddushin 5B

חופה שגומרת ⁠– אינו דין שתקנה? מה לכסף, שכן פודין בו הקדשות ומעשר שני. ביאה תוכיח.

is it not logical that a wedding canopy, which completes the marriage, since it entirely removes a young woman from her father’s authority, can effect acquisition for betrothal on its own? The Talmud rejects this claim: What is unique about money is that it can effect acquisition in many contexts, as one can redeem with it consecrated property and second tithe. This is not so with regard to a wedding canopy, which is ineffective in effecting any acquisition other than marriage. Therefore, in the case of betrothal, money can effect betrothal while a wedding canopy cannot. The Talmud counters: The halakha of sex proves otherwise, as this act serves to effect acquisition of a woman despite the fact that it is not a valid mode of acquisition in any other case.

מה לביאה, שכן קונה ביבמה. כסף יוכיח. וחזר הדין: לא ראי זה כראי זה, ולא ראי זה כראי זה, הצד השוה שבהן: שקונין בעלמא וקונין כאן, אף אני אביא חופה שקונה בעלמא, וקונה כאן.

The Talmud counters: What is unique about sex is that it effects acquisition of a yevama, whereas a yavam does not acquire her via a wedding canopy. The Talmud answers: Money proves otherwise, as money cannot be used to acquire a yevama, and yet it is a valid mode for acquiring a woman. And the derivation has reverted to its starting point: The aspect of this mode, money, is not like the aspect of that mode, sex, and the aspect of that mode, sex, is not like the aspect of this mode, money. Their common denominator is that they generally effect acquisition, and they effect acquisition here, with regard to betrothal. Likewise, I will bring the mode of a wedding canopy, which generally effects acquisition, rendering a woman as married, and therefore it should also effect acquisition here.