Source why ביאה is not an option for שליח קידושין

Question

This might sound a bit silly, but "that's the Torah and I need to learn it" (R"A).

1. A woman can be Mekudeshet in three ways which Halachah-wise are all equal (neither makes a better Kiddushin).

2. A messenger can do the work on behalf of the sender either by money or a bill, but not ביאה (Kesubos 74a).

3. We also hold that there does not have to be proof of the מעשה, only ראוי למעשה, so the witnesses only see them entering and exiting (and zipping up). So theoretically we could allow the messenger to seclude with the bride on behalf of the groom without doing the מעשה.

To remind that that ביאה is for Kiddushin only and is different from the second ביאה for Nissuim.

I need a source that speaks explicitly about why not the last. Thank you.

Answer 1

R' Elchonon Wasserman in Kovetz Shiurim (Kesuvos 74a) brings this question from R' Akiva Eiger, and explains how the Kiddushin of Biah is fundamentally different from Kessef and Shtar, and why that has ramifications with Shlichus and Tnai.

Others (source to follow in edit, bli neder) explain that Kiddushei Biah is not just an arbitrary maaseh kinyan (like Kessef and Shtar would be); it's power lies in that fact that it is a haschalas ishus, which would only be if the husband (the Mekadesh) would perform the action. The Biah of the Shliach is not just performed by the wrong person, but is the wrong action.

Answer 2

In a lecture at YU's Gruss Kollel (I think this one) R. Aaron Rakeffet described how this question was asked by the high school students in the 1960s, and the answers they were given:

I always tell the story of what happened in YU in the early 60s when the rebbeim were set up by the students. Every rebbe got the same question. We were teaching Masechet Kiddushin, I apologize to the ladies here for this example, but it's a true story.

So if you've learned Masechet Kiddushin you know you can marry a woman in three different ways: kessef, shtar, biah. Then you have Perek Sheini – that you can marry her, you can send a shliach to marry her. So very innocently, I get the question, and this goes back maybe to '62 '63, I get the question: "Rebbe, can you send a shliach to be mekadesh a ishah for you al yedei biah?" You follow the question?

So I look at the class, baruch Hashem I had seen chiddushei Torah from R. Shurkin of Chaim Berlin a few weeks earlier, and he asked the very question. And he says over from R. Chaim, and this is typical R. Chaim, there are two different types of acquisition – kinyanim – one kinyan is the universal form: the way I buy a field, the way I buy a car, whatever you can sell, there's universal forms of kinyan. Buying an ishah is no different. But relations between a man and wife by definition is ishut, it's not universal. So when can you make a shliach? When it goes with universal concepts. Just like I can make a shliach, Sam, to buy a car for me, to buy a house for me, I can make a shliach to buy a wife for me. But when it comes to dinei ishut no shlichut is possible. And I gave that answer. And that's R. Chaim.

(My transcription, fillers removed)