Christian bakers are not asking not to participate in a homosexual wedding. Or if they are, they've been poorly advised as to rhetoric, which is choice of the right argument.
Since homosexual marriage cannot exist, marriage being what it is, bakers and photographers are asking not to be morally pistol-whipped into agreeing that reality is what it is not.
The Christian gun shop owner, and the man he sells to, on the other hand are each making their own choices on their own responsibility in this fallen sinful world.
And by the way, if the gun-buyer is not a murderer but rather uses his gun to protect a homosexual couple on their way to city hall, does the gun shop owner get credit for saving gay lives? Also by the way, will the man who made this adorable and specious poster raise a breath in protest of gays being thrown off roofs in Syria? I'm guessing not.