You are making judgments on what entire groups of people really think. Based, as you admit, only on what you have witnessed. If choosing when to have sex is a valid option, then choosing to have a ceremony to honor and acknowledge that choice is just as valid as having a ceremony to honor a second poly husband.
I have to be honest, I seriously doubt any purity ceremony really involves people talking about how 'dirty' sex is. I suspect this is your interpretation of what they think, rather than what they actually said. Certainly you know different individuals than I do, but I also know plenty of people who believe in waiting until marriage, and it is not about sex being 'dirty,' but about sex being so sacred and beautiful that it deserves a special consideration, and is reserved for a spouse. Whether anyone agrees with reserving it fo a spouse, it is simply wrong to claim that all these people, or even most of them, believe it's 'dirty.'