Having sex in her period? Not that bad, but still a bit messy.
But if I had some girl talking like this girl of yours I'd answer something else:
GIRL-"well a boy would be left out, while a man would be right in. Are you a man?"
Answer A: "Stop talking and get a towel...what are you doing? I didn't ask for condoms."
Answer B: "Yes, but you know that men do whatever they like with girls. So today I'm/I'm not in the mood for it.
ANSWER C: "well, normally I have nothing against girls dripping rotten blood, but today I'm not in the mood. Don't worry I have a plan B."