What is 'too' dangerous? What is simply dangerous and what is an acceptable danger?
A lot of commenter says that the whole human space exploration is too dangerous for the man who dare it, but this is nonsense. One of the basic principle of the human race is (or should be) that everyone is free do to everything he wants without harming anyone else. There are people who chooses dangerous sports for funny and people who does dangerous job for live. Anyway nobody ever questioned the motive of a firefighter or a bomb-squad technician: if there are people willing to risk for the sake of others, why should not we let them do?
There are several aspect to consider in a long trip to Mars, but non of these is 'too' dangerous. First of all there is radiation exposure. It is true indeed that a Martian crew would not be protected by atmosphere from space radiations, but the members would also exposed to less radiation coming from terrestrial source. Earth is a continuous source of radiation from various kind of rocks, so we are normally exposed to the so-called radiation background. This is something we evolved to live with and something we need to live. A part of our immune system evolved to repair damage from natural radiation exposure, and the lack of radiation causes sickness, exactly how too much radiation do! Of course that does not mean that a travel in the space is good for your health but simply that a single trip to Mars and return provides a radiation dosage comparable to the one accumulated in a few years of flies of a professional transatlantic jet line pilot. This is still a huge amount, but nothing that denies a healthy person to complete one or two trip to Mars in a lifetime. All in all it is much more dangerous to smoke for 40 years than go to Mars: the risk of lunger cancer is very elevated in the first case. A really dangerous situation during a Mars voyage would be solar flares, brief and powerful solar events that carry various kind of electromagnetic waves. Fortunately they can be predicted hours before they reach the ship so it is possible for the crew of a spaceship to take refuge in a specifically-designed heavy shielded part of the ship.
Another danger would be isolation. It has been said that a crew would go crazy of being restricted in a small space isolated for months. Again, history of human race told us differently: in the past centuries people were isolated for longer period of time with none of the modern commodities and they still accomplished spectacular results like Earth circumnavigation. There is no reason why a group of well-motivated and equilibrated people could not live and work together for the duration of the mission without losing their minds. It should also be considered that a Mars crew would have continuous (even if not live) communication with colleagues, expert technicians and beloved ones on Earth. This is not a children game, but human factor is in no way the weakest link of the mission, on the contrary is probably the stronger.
Finally let's have a look at the medical problems. It is obvious that the crew would be millions of kilometers away from any advanced medical facility, but this is not crucial to the mission. The members of the crew would be selected among healthy people and they would not be exposed to any pathogen in the sterile spaceship environment. During the journey to the Red Planet they would not likely suffer from any serious injury, since they are just doing lightweight works. There are also other space-related medical problems that could arise during the trip, but they can be minimized and even avoided completely taking two crucial measures. First, a small artificial gravity should be provided to the crew: this could be obtained making the whole spaceship rotate, this is not difficult at all and it can be achieved in a quite simple way. The second measure would be to impose heavy physical exercises on the crew: a small part of the ship can be devoted tho this with just a handful of machines that are normally used in any common gymnasium. Physical exercise is good for the body and it is also very useful to keep your mind focused and for team building.
I have just briefly written about the major foreseeable danger related to similar mission, there are thousands of details to talk about and a lot of scientists still have to do the hard work about this, but the principles I just exposed are indeed valid. Going to Mars is not a deadly business, is a risky one. We used to say that nothing great was ever accomplished without risks and human race is just meant for great things.