I could just say (hypothetically. I really don't care if it happened or not) that the holocaust did not happen. It was made up. Does that offend you? Too bad. There is no way either of us can prove/disprove either the holocaust or the binding of Isaac. What I'm saying is that just because one person believes or disbelieves in something, it does not make it factual. You might say I'm brainwashed into disbelieving the holocaust, but the religious fundies might say the same about you.
When you are part of a society, you have to be mindful of everyone's beliefs. You should not let your personal biases interfere with what should be deemed offensive. If it offends a segment of society (even if another segment deems them to be idiots), it should not be allowed.
Would you approve of a game that lets you fly planes into buildings intentfuly? I think it's crazy people are still so offended by 9/11. However, since it offends people, games like those should not be made.
edit: I'm not easily offended--secular themes like sex, violence and drugs don't bother me at all. I'm not offended by religious themes when they are tastefully done-- Civilization 4 does really well here. But when you are making fun of a person's beliefs (however absurd you may think they are), it's going too far.