What it comes down to for me, is manipulation. If I am close enough to someone to be invited to a wedding, am I not close enough to know when they go through major life milestones? Regardless of whether the religious or social aspects of a wedding are more emotionally meaningful than the legal benefits, there is a difference between "not making a big deal about something" and doing it in secret.
If I am already aware that two people are married - as a pp mentioned - and they invite me to their BWW, I dont' care what they call it and I will go. But if someone is actively concealing the fact that they are legally married, I have to ask, why? What are you trying to get me to do, that you think I would not do if I knew the truth?
This actually happened to me about 8-9 years ago. DH's family was all raised in, and adheres with varying degrees of commitment to, the same faith tradition. There is definitely a common understanding about weddings/marriages and the importance and traditions thereof. They LOVE weddings. Everyone will travel hundreds or even thousands of miles to attend a family wedding, and they are often used as de facto family reunions.
They also have a binary view of marriage. There is no concept of being legally married as something separate from "married in the eyes of Diety" or "in the eyes of the Church", or anything like that. You are or you aren't. Regardless of what an individual couple may feel, in this family this is a well-established understanding to 50% of the guest list.
DH and I were invited to Cousin N's wedding, several states away. It was a big trip, we ordered a nice gift to be sent to their registry address, and went. Everyone very happy and excited.
At some point during the ceremony, we all realized that the officiant was not administering vows. He was, instead, twisting the words of the traditional religious ceremony to be a blessing of the vows which the HC had already exchanged. "HUH?" The family is looking at each other in extreme confusion. The bride is furious, glaring at the officiant.
They stop the ceremony and the groom takes the officiant aside. When they return, they finish the ceremony, with the officiant giving pointed looks at the HC, and great emphasis on the words, "DID you promise..." I thought the bride was going to throw down her bouquet and strangle him.
I have no idea what went down with the officiant, whether they lied to him about already being married and he found out at the last moment, or whether he intended to do a fake wedding and backed out. Come to find out afterward, HC had been married for over a year and kept it a secret from the entire family. I'm not even sure if the bride's parents who footed the bill knew, possibly they did.
Ultimately, it was so senseless because if they had just announced that they eloped, everyone would have sent the same presents a year earlier, and if they threw a postdated reception everyone or nearly everyone would have come anyway. And instead of having a fun party, they spent the whole time being mad and embarassed because they were lying and got exposed.
We used to think of Cousin N as "fun and flaky Cousin that we wish we saw more often". Now we think of her as bizarre manipulative lying Cousin, who we are happy to see less of.