Polygyny is not a religion, but rather a type of marriage recognised by God, Jesus Christ, The Bible, and Christianity. Just because polygyny hasn't been taught or practiced for many years doesn't negate it. What's popular isn't always right and what's right isn't always popular.
"1. A religion or religious sect generally considered to be extremist or false, with its followers often living in an unconventional manner under the guidance of an authoritarian, charismatic leader."
- I haven't met anyone that has considered Christianity to be something that is extreme and it's definately not false. We Christians live in a normal manner under the guidance of our Heavenly Father and his son Jesus Christ, but not a normal human being.
"4. A usually nonscientific method or regimen claimed by its originator to have exclusive or exceptional power in curing a particular disease."
- Christianity is definately not scientific, although as time goes on science does gradually prove Christianity more and more true.
"1. Obsessive, especially faddish, devotion to or veneration for a person, principle, or thing."
- Christianity is definately not a fad. Seeing as how it's been around for thousands of years and all.
I highly recommend that you attend one of your local churches to find out more detailed/personalized information about Christianity.