Our perceptions of sex are affected by our cultural, religious, and historical family beliefs. Many of our misconceptions about sex can be traced back to what great-great-grandma taught/didn't teach her kids about sex.
Mormons were not always Mormons. Joseph Smith organized the LDS church only about 180 years ago. Even at that time, they were just organizing and beginning to get revelation about what church doctrine should be. Even today, our beliefs and practices are still being refined.
Before our ancestors were Mormon, they most likely belonged to another church. Even today, we have converts to the church who were Catholic, Protestant, Baptist, Methodist, Hindu, Muslim and many more.
When converts come into the Church culture, they bring their family beliefs and superstitions about sex and marriage with them. Many of those beliefs are incorrect. I know my parents had some pretty interesting beliefs about sex, and they were beliefs that were contrary to what the LDS church teaches.
Just for fun:
How far back can you trace your family's religious history? What did those religions teach about sex? Do they match your current beliefs about sex? Why or why not?
How have these beliefs and traditions affected your sexual relationship with your spouse?