Beautiful Breasts

 Daf Yomi just learned the following Gemara (כתובות ע"ה)

תָּנֵי רַבִּי נָתָן בִּירָאָה: בֵּין דַּדֵּי אִשָּׁה טֶפַח. סָבַר רַב אַחָא בְּרֵיהּ דְּרָבָא קַמֵּיהּ דְּרַב אָשֵׁי לְמֵימַר טֶפַח לִמְעַלְּיוּתָא. אֲמַר לֵיהּ רַב אָשֵׁי: גַּבֵּי מוּמִין תַּנְיָא, וְכַמָּה? אָמַר אַבָּיֵי: שָׁלֹשׁ אֶצְבָּעוֹת

 Rabbi Natan Bira’a taught: A handbreadth between a woman’s breasts. The amora’im have a dispute concerning the meaning of Rabbi Natan Bira’a’s statement: Rav Aḥa, son of Rava, thought to say before Rav Ashi that this means a handbreadth between a woman’s breasts is perfection and considered beautiful. Rav Ashi said to him: This baraita is taught with regard to blemishes, and it means that if her breasts are separated by a gap this wide, it is a blemish. The Gemara asks: And how much of a gap is considered normal? Abaye said: The width of three fingers.

This Gemara raises a number of questions:

1. The only way to see the distance between a womans breasts is to see her topless. When a woman is wearing clothes there is no way to see this. So how did Abaye know this? Did he go and stare at naked women? How many naked women did  he see besides his wife? How is this permissible?
2. Beauty is in the eye of the beholder, for some men a gap of 3 fingers is beautiful for other men it may be either smaller or larger. How can the Gemara set a rule on something like this?
3. How did they come up with these exact measurements? Did they go and measure women's chests? Does the average man when he sees beautiful breasts have any idea how far apart they are?
   
4. What is the relevance? Why does the Gemara need to tell us the definition of beautiful breasts?