You lose your "fratginity" the first time you have sex with a guy in a fraternity. However, you have only lost your fratginity in that specific house, you remain a fratgin in all remaining fraternities.
"Shit, I was so blackout last night, I lost my fratginity at PKA! I had sex with Jethro!"
My first winter in Minneapolis I was a shivering little waif. But now I've lost my friginity and can withstand weeks of below-zero temperatures without batting an icicle-encrusted eyelash.