I don't like blurred lines between casual fun and games and something involving such stark, serious reality as firearms. I also don't like making a game like that out of combat; I also dislike the so-called "first person shooter" computer games for the same reason. I have no problem with more abstract combat simulation games.
I don't have a problem with the guns themselves, so much. I mean, I disapprove of toy guns in general, but I don't hate them. Their proper sphere in my world is at the gun range, shot just like a gun. My daughter has a Red Ryder BB gun, and before she graduated to .22 rifles, she shot her BB with eye and ear protection at the rifle range. We treated it as a weapon just like my SAR-1 or Mauser rifles, and she enjoyed it more, not less, for the seriousness with which we handled it and the responsibility she knew I had entrusted to her. We most assuredly did not run around in the woods, shooting it at people.
Softair and paintball games run against the grain of everything I believe about firearms and combat. That's why I hate them.