The scene: my first apartment, age 20. Girlfriend (an Irish redhead, lithe, beautiful...) wants to come over and celebrate newfound freedom with a wild night of lovemaking. We plan for days; I stock up on Karma Sutra products and condoms; she buys bra and panties special to the occasion.

She knocks on the door, and I greet her warmly. She looks at me as she enters with some trpidation, and I ask her what's wrong, as I close the bedroom door behind us.
"I, ummmm... I got my period." She winces.
"Oh!" I hold her hands and step back. "We don't have to... we could just go to Blockbuster..."
The look of disappointment is immediate. "We could..."
Any suggestion she has at this moment is very welcome. "...yes?"
"My big sister says it actually helps her... having sex when she's got her..."
"You still want...?" I look eager, and ignorant-- she realizes I'm missing the point and she'll have to spell it out."
"There'd be a bit of blood."
"Oh... that's alright with me, if it's--" She starts hastily stripping right there and I grab her, rushing her into the bedroom.

An hour or so later, after much teasing and kissing, I am about to cum inside her. I pull out, gripping the condom so that I can collect the sperm without spilling on her. As I raise up and the coverlet falls away from her body, I see the blood. My shaft and belly are smeared in a thick, dark blood. Her crotch looks like a vicious stab wound, with blood oozing from the condom to where I 'stabbed' her.
She covers her eyes, embarrassed and mortified, but I'm still staring, satisfied with this mock image of rough sex. Curling my lip like Billy Idol and breaking into my best Spinal Tap impression, I say, "None more black."
We laughed for hours...