Weddings are usually very adult events and most couples don't find it particularly endearing to pause their vows for a screaming toddler's temper tantrum or watch their maid-of-honor run out to change a diaper during the reception speeches. Open bars, dance floors, and ceremonies that require silence and attention are not exactly child-friendly activities. While some people find ways to include children into wedding festivities with flower girls, ring bearers, "ice cream bars," or play rooms, if a couple asks specifically for a child-free event, parents are expected to respect their wishes.
That being said, if you choose to exclude children, you must accept that many parents will simply be unable to attend as finding childcare, especially for a destination wedding, can be incredibly difficult. So, when a frustrated woman decided to consult the moral compass of the internet otherwise known as Reddit's "Am I the as*hole" about her sister's request to be photoshopped into photos of her child-free wedding she chose not to attend, people were eager to weigh in.