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.'
A purity ceremony is degrading and disgusting. Interesting. Would changing the name be acceptable? What name would make the concept acceptable?
Again...I simply find it interesting that people who would be highly offended at someone else labeling their choices 'degradging and disgusting' are so willing to label others. Doesn't this defeat the idea of tolerance?
If you don't want to have a purity ceremony, don't have one. I'm a little surprised it makes that much difference to anyone here what such a ceremony is called. If we're sure of our own choices, it hardly matters what anyone else thinks.