[elmokki]

Steam DOES NOT require you to have your game use it in any way for non-Steam distribution. Steamworks API is rather simple and you should very easily be able to make the Steam version for Steam and a non-Steam version for whatever other ways you distribute it with. Of course it is extra work, but seeing Steam's market share it's financially probably easily worth it in nearly all cases.

There are many games that can be bought from Steam or can be bought from elsewhere and played without Steam.