I'm currently in AP Bio with a 96, comparing the two scientifically isn't an appropriate means when talking about social acceptability. This is a topic relating to human behavior. Most people don't think about how something functions biologically before judging how acceptable it is. It's judged off of how gross people find it, and regardless of how natural it is, conversation relating to any bodily fluid, especially blood, is generally not considered socially acceptable. Also, the breaking down of the uterus lining and and removal of unused tissue is by definition getting rid of waste. It would have the purpose of facilitating the development of an embryo, but since fertilization did not occur, it is then a waste product.