This was a few years ago, but my girlfriend at the time knew I loved watching her pee (especially in public).
One night we were in bed and I was going down on her. She wanted to stop so she could take a quick pee. I suggested that she hold it in for as long as she could so I could try something.
10 minute's later or so she's starting to squirm and struggle to hold it in.
I bobbed my head up and told her I wanted her entire bladder emptied down my throat, that way I could pee it out for her. She got pretty turned on by that thought!
I'd only just got a nice suction around