Why is this Russian expression an idiomatic example of mutually exclusive things?

Question

The Russian idiom "и рыбку съесть, и на хуй сесть", whose literal meaning is "to eat fish and sit down onto a dick too," is an idiomatic way to say that your interlocutor is talking about mutually exclusive things, like "to have your cake and eat it too." Here is a random example from an Internet conversation:

X: Хочу похудеть, но не люблю заниматся спортом!!!

(I want to lose weight, but I hate doing exercises!!!)

Y: так не бывает, ты что хочешь рыбку съесть и на хуй сесть????НЕТт, а значит ходить тебе жирной до конца дней твоих, просто глупый вопрос.

(It's impossible, c'mon, do you want to eat fish and get a f*ck too????? NO, and this means you're gonna remain fat for the rest of your life. It was just a stupid question.)

The usage is like that of a non-obscene analog, "влезть на ёлку и не уколоться" ("to climb a spruce and not get scratched").

My question: Why would it be mutually exclusive to eat fish and have sex? What's the logic behind that?

I understand it's hard to eat fish in the very process of having sex, but the idiom is clearly not about that. It's composed by using perfective rather than imperfective forms of verbs and is thus like "to get food and a f*ck" or "to have a dinner and sex."

I tried to find an answer on the Internet, but to no avail. The corresponding Wiktionary page says that the idiom is a rephrasing of "либо рыбку съесть, либо на мель сесть," which literally translates as "either to eat fish or to run aground" and is a figurative way of saying "all in," but that doesn't explain the logic of the rephrasing, that is, why it's considered mutually exclusive to eat fish and have sex. Russian SE has a couple of related questions (link1, link2), but none of them asks what's incompatible in eating fish and having sex.

There must be some Russian logic behind that, and I humbly hope that Russian speakers can shed some light on it.

Answer 1

No special logic behind it is intended.

"Либо рыбку съесть, либо на мель сесть" is not about two mutually exclusive options, it's about a typical "win or lose" situation which is always an option. When paraphrased obscenely the phrase keeps its rhythm and acquires a very funny meaning by the addition of хочешь and the substitution of “и ..., и ...” for “либо ..., либо ...” – apart from a funny sexual reference it is now turned into the direct opposite of the original meaning, irrespective of how exactly opposite it is: if addressed to a woman, it becomes to mean “to want a win or double win” which is impossible, there is always a chance to lose; and if addressed to a man, it becomes to mean “to want to win or double lose” since a man is not supposed to want having sex with another man, that is why one cannot want both, which is funny because of the absence of logic in such a desire. Naturally, in each communicative situation it can get turned this or that way and show different edges of its funny meaning.

Answer 2

The original saying was used to describe an all-or-nothing personality. Someone who will either get results or dies trying, either “catches the fish” or “runs aground” trying. This website provides a quote from “Девятый вал” by Григорий Петрович Данилевский:

― Ох, барин, берегись,— говорили ему его хозяева: — дело берешь трудное; с ним либо рыбку съесть, либо на мель сесть.

В.И. Даль also mentions this saying among others about “Courage, Bravery, and Cowardice”. This tells us, that the saying was already known as an example of a kind of all-or-nothing thinking.

As for the currently popular argot phrase, it just an example of the culture of русский мат expanding through adapting and mutating already existing idioms. “На хуй сесть” simply rhymes with “рыбку съесть”, and I don't think there is anything more beyond that.

Answer 3

The logic is that to eat a fish (supposedly hanging in front of you) you have to move your torso forward while to sit on a dick you have to move your torso back. So, you hardly can do the both things at the same time. At least, it is how I understand it.