While your first statement does have merit, it ultimately doesn't apply in this situation.
Space is a whole 'nother ball game no matter which way you look at it. While the oil and gas companies may call the shots in politics, they are the ones actually blocking something like this happening. Politicians don't really care about spaceflight, they care about pork for their districts and that's it. You can see this by looking at the launch costs for the last 40 years or so.
Spoiler: They haven't changed. Keeping the cost to space high puts the need for a high bar of profit to do anything there(if you're a company) which is why we have almost zero space commercialization now.
Today, there is absolutely zero reason to colonize mars other than "We need to as a race" or "It would be a great challenge", etc. Believe me, I'm a huge proponent of space travel, but it all costs something and currently the cost is too damn high to make any mission like that economicly viable.
When it is viable economically, you won't be powering such a colony with fossil fuel based products. It's ludacris to expect to ship what would amount to millions of tons of the stuff over time to power their generators once. What I mean by this is, once you burn the fuel, it's gone. There is not any oil on Mars, so you'd have to import it at from Earth. By looking at the time and cost investment of a single shipment to Mars, solar is orders of magnitude more economical. Honestly, to send your first colonies to Mars and expecting them to live off of fossil fuels and not solar for electricity, is sending them on a suicide mission. It can't happen.
As far as NERVA and nuclear rockets go, they can go radioactive and dangerous but not necessarily. It depends if the reactor runs open cycle and dumps the energic neutrons into the exhaust. If it's closed cycle, then this doesn't happen...but after time your core gets more and more radioactive and also loses efficiency(heat) due to halflife of your fuel. Where to put the old cores, and how to replace them is one of the problems facing nuclear engines today. I say one because stigma towards nuclear anything is at an all time high, and no one likes the idea of nuclear reactors flying over our heads...even if they aren't started/radioactive until they reach space. It's a tough sell.
This is not entirely true and also depends on the type of nuclear engine you're running. For the record, KSP has it wrong. It gave Nervas high ISP but low thrust to balance the engine for gameplay reasons. A real nerva(using hydrogen as fuel) has great ISP, around 1200s, and pretty good thrust comparable to chemical rockets.
I don't want to cherry pick quotes, but if you read this wiki you can see that various designs have varying ISP and thrust values, but ultimately once in space they are miles better than chemical rockets due to their much larger ISP values, and the fact that ISP increases your efficiency exponentially due its logarithmic nature.
All you need to know, is that for our age(baring any serendipitous physical breath throughs such as the EM drive actually working or discovering warp) these nuclear engines are the best engines(efficiency wise) to launch us into space and get us to nearby destinations. However, politics and common sense are a pretty large obstacle to overcome. Chemical rockets will reign supreme for access to LEO for many years to come, but once there the switch to electric propulsion is on it's way. Whether we power our manned missions with nuclear reactors(in which case it's better to go NERVA so that you don't have to worry about radiating your waste heat) or with extremely large solar panels for power remains to be seen.