1. A term used to qualify man's penis when said man likes having things shoved into his urethra.
2. Also used to refer to the person (usually connotates that the person is gay).
If you're lucky enough, you can probably find a video about this (there is one around on StileProject under the name bagina5.mpeg).
"Holy shit! That guy got a dildo shoved up his urethra!"
"Fuck! He's been graced with a bagina!"
"You're a fucking bagina!"