Actual Ex-Mormon who attended BYU here: Soaking was never a thing, I have only ever heard about it on the internet or literally in the context of Mormons laughing about non-Mormons believing in Mormons doing such things (yeah, they're meta about it).
What is an actual thing is Mormons getting married super early (for a multitude of reasons, one being the horny). Easily over 70% of the students I knew were married by the time they were seniors in college.
I attended BYU-I in person for three years. There was a lot of dumb s### that happened there, but I can say with confidence this wasn't one of them. To not be a buzzkill though, I'll share an actual saying that people use around campus: "BYU I do." Because like 80-90% of students there expect to be married by the time they graduate.
I'm a Mormon, and this just can't be real. Sexual contact is sexual contact. How would people told to leave enough room for a Bible between them while dancing think that this would be okay?
I'm convinced this rumor exists just because people want it to be true.