First, astronauts are NOT human guinea pigs. They are temporarily in space to perform work - and then they come home.
And there have been absolutely no experiments done on the effect of microgravity on humans during the course of that work?
Nobody said this was gonna be easy. Tremendous difficulty. But it is far more feasible - far more - to create long term habitation space vehicles than it is to try and make-pretend that Mars is just a smaller Earth requiring a bit of tweaking and tuning. Mars is inhospitable for human life, animal life, plant life. That's how it is.
What an odd argument. Mars is inhospitable for human habitation, therefore it must be easier to go somewhere that is even worse. No-one is suggesting we will be happily skipping around in daisy fields on Mars. But on Mars, or any rocky planet, there are some big advantages over simply floating around in space.
Firstly, an atmosphere. In space you have to have a completely self-contained box to live in and take all your own atmosphere with you. On Mars you have to have a self-contained box, but it doesn't have to be as strong and you can get some gas from outside.
Secondy, radiation. In space there's a hell of a lot it and you need all kinds of shielding to stop it. On Mars, not so much. Even a thin atmosphere and very weak magnetic field is better than nothing. It's also very easy to just dig holes and build underground.
Thirdly, safety. If something goes wrong in a spaceship, you're pretty much screwed. If you lose some air, it's lost for good. If some equipment is damaged, you can't replace it. On a planet, you would be making your own air from the existing atmosphere so if you lose some you can replace it. If something is damaged you could repair or rebuild it using local materials (although I admit this would depend a lot on the type of colony. A small research post would have similar problems to a spaceship, but permanent colony would likely have manufacturing capability). In space there are also problems with impact from micro-meteorites (and not so micro ones if you're unlucky). However, while these aren't a problem on a planet there are other things such as dust storms that could cause similar problems.
Finally, gravity. Planets have it. Yes, Mars' is only about 1/3 of Earth's, but that is a lot better than nothing. So far we have no data about how much gravity is necessary for human life. Maybe 1/3 gravity is low enough that it will cause unsolvable problems for long-term living. On the other hand, maybe it won't. The important point is that it is there and it is free. In space, you have to make your own gravity. Cetrifugal force is all very well, but that means energy to make it spin, bearings that wear out and serious problems in steering dues to angular momentum. Also, to get 1G over a whole human body requires very large structures, certainly bigger than most asteroids, otherwise you will only get 1G at your feet while the rest of your body has much less, which pretty much eliminates any point of having it in the first place.
Basically, to live in space you have to do everything from scratch, while to live on Mars you have a solid foundation to work with. Maybe space living will eventually prove to be more feasible, but to pretend that it is such a cut and dried case as you make out is just silly, and ignores many of the best minds' thoughts and work on the subject. It is also silly in the extreme to pretend that everyone who disagrees with you either hasn't read or doesn't understand your argument. This is a tactic usually only seen from woos and not from those trying to actually have a sensible discussion about something.