Why was this thought about Darwin considered to be stupid?

Theory's a theory's because it's a thought, an idea. We don't actually know what happened, but we assume based on research in a controlled environment. For all we know there may be many ways. Life is created in many different ways through many different mediums, it was just inferred that the easiest way was water mainly because of the composition and amount of carbon in the atmosphere, cells are different than humans or animals, all they need is a little carbon and an environment and they multiply 2X2 4X4 6X6 and so on. how the first life form came to be may never actually be known, but we can sure as hell try to guess.

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"Another living being must have been there at the same time (stupid)."

It is stupid because it's not true. You don't understand autotrophy. The first organisms were autotrophs.

"It must have been hermaphrodite (stupid)."

It is stupid because the first organisms didn't have sex. They reproduced by binary fission as a vast number of their cousins do until this day.

"It must have been a cannibal (stupid). "

Once again you are demonstrating your lack of knowledge on the subject of autotrophy.

"Are these thoughts really that ridiculous even if they can't be proved one way or the other?"

All of the thoughts you've enumerated here are absolutely ridiculous and can be proven so via common knowledge. Take a Biology 101 class.

Abio-genesis is a stumbling point for all Evolutionists, including Darwin.

The issue is ignored, side-tracked, red-herringed, and overlooked. They may tell you when it occurred, or where it occurred, but never how.

They will answer your Qs easily. A photosynthetic bacterium, that reproduces by cell division.

The problematic issue is this; "abio-genesis" is impossible. The Law of Biogenesis and the First Law of Information say it is impossible. There are no known exceptions to these 2 laws, but abio-genesis breaks both of them.

Scientists are confounded by this problem so they say "its not part of evolution" or make wild speculations like "it happened on the back of crystals".

To summarise, abio-genesis of the first life form is naturalistically impossible. Where can impossible events occur, "no-where". When can impossible events occur, "never". Unless of course they are the actions of a super-natural being, ie God.

Life pre-dates sexual reproduction by many, many years. Having a "mate" around would've been unnecessary. Check out the link I provided if you would like to know more.