Even if you know that there is no such thing as original sin and your partner agrees that a baptism is just a tradition but it's important to their family as it is a tradition they feel is necessary. Would you allow it? Would you go to the event? Would you be open with your growing child dispel the nonsense that the child would hear when they are around their christian grandparents? Anyone deal with a partner that's reasonable and doesn't practice the religion of their birth but their family does?