See, there is that "simple solution" always proposed in HN comments that solves everything and sweeps away every possible objection. Just send robots.
Look, the GP went beyond "colonizing" into "a multiplanetary species". We have already landed and walked on the Moon, we've established an extremely safe and stable outpost in LEO, but we haven't even colonized Antarctica!
Think about that for a few. We can't colonize Antarctica. It is not far away; it has plenty of breathable air; it's... replete with water, if you will. But still, there are only a few scientific outposts that are unsustainable without constant resupply and staff changes. That's not a colony.
I don't believe we could colonize the Moon. I mean, the astronauts could walk around after landing, and the supply chain isn't absurdly long, but to sustain a Moon outpost you'd need a perpetual supply chain, sending up needed provisions.
I don't know about your definition of "colonize" but, generally on Earth, that means putting down roots and establishing a rather self-sufficient presence that happens to consist of foreigners. How does a colony of ants get established? They find food and resources nearby; they begin mating and reproducing; they build with what they find. The ants are not smuggling in breathable air, food and water over 10 miles away.
The supply-chain to Mars is ~6 months. I mean, you can fly humans there in about 6 months, every 26 months. Now imagine that you have a "colony" or "outpost" on Mars that needs perpetual resupply. Can you launch 26 months' worth of provisions in the short window every 26 months? Can you launch the provisions during the worst-case window, when it will take much longer to arrive? Are you going to send a bunch of provisions up with the robots in advance?
Those resupply logistics are just nightmarish, and they are daunting to envision, even if everything goes right with every single launch and voyage. The margin for error is nil. The opportunity for rescue or emergency escape, is nil. Again, send humans to Mars, but don't expect to get them back in one piece!
So, I contend that even with spinning capsules, and advance-team robots, and even with some magical method of extracting resources for fuel, a Mars colony or self-sufficient outpost is infeasible with current technology, as well as any foreseeable future technology. Sure, set up a "Moon base" but it will always be more difficult than Antarctica, and never self-sufficient. Rescue and escape from the Moon are likewise unlikely.
The thought leaders who are promising "multiplanetary species" are leading you down a primrose path. They're encouraging fantasy scenarios so that young kids will enter STEM programs and pursue science career paths. They solicit donations and support for tech programs that are, ultimately, just part of the military-industrial complex.
But it's all borne on the wings of sci-fi imaginations and not gritty reality. I was just chatting with my parents about how the ISS smells, and how chaotic it is. It's worse than a submarine. It's sending our best & brightest to camp in a hoarder house. They've got a chance of survival if things go wrong up there. On the Moon or Mars, they wouldn't. They won't be smiling, and they won't be growing potatoes.