Russia has recently approved a law which puts a ban on "propaganda of non-traditional sexual relations". For instance, just saying that you are gay in public is now illegal in Russia. This is a clear violation of the Olympic Charter, "The Fundamental Principles of Olympism", rule 6, which states: "Any form of discrimination with regard to a country or a person on grounds of race, religion, politics, gender or otherwise is incompatible with belonging to the Olympic Movement."
Therefore the IOC has no other option than to obey their own rules and exclude Russia from competing in, and organising The Olympic Games.

Link to the Olympic Charter: http://www.olympic.org/Documents/olympic_charter_en.pdf