It got picked up from the Ute Trolls, when BYU got into the Big 12, and perpetuated from there, by the other Big 12 school trolls. I had never even heard it referenced in any messageboard until about 3-4 years ago.
As for whether it is a thing at BYU, that is just another false stereotype, just like the whole eloping to have sex, and then getting an annulment the next week, rumors. People have been fooling around in every faith, forever, and teenagers trying to stay within the rules, have always tried to justify those limits to keep their so called “purity”. It happens with evangelicals, Catholics, Muslims, Jews, and every other faith that believes in saving themselves for marriage. I could go into more detail, but don’t want to get a CB warning. Lol There are a lot of things that people will come up with to still be able to claim on their wedding night, that they are “virgins”