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.