When they get back to her place, she says, "I didn't want to tell you before, but I've got a fetish. I'd love it if you fucked me with your big toe."
The man, an agreeable sort, goes ahead and does it. Has a pretty good time. But a few days later, he notices his toe is red and inflamed, then it starts itching and burning.
He goes to the doctor, who seems very confused. But after a few tests, he says, "I don't know quite how to explain it, but you seem to have toe syphilis."
The man says, "Well I'll bet that's about the strangest thing you've ever seen."
The doctor responds, "Oh, you'd be surprised. I mean, just the other day I had a woman in here with athlete's vagina."