"Wait!" Bobby say suddenly.
"What's the matter?" responds Katie.
"You're a were-mermaid. Will sex with you turn me into a were-mermaid too? Or a were-man?"
"I don't know" admits Katie. "I can't exactly look my condition up in a diagnostic manual."
"What about your past boyfriends?"
"I never let any of my previous boyfriends find out."
"Oh." Bobby hesitated, considering the risk. As much as he liked Katie as a mermaid, he wasn't sure if he would be okay having a monthly tail of his own. And he definitely wanted to stay male.
Katie rubbed her tail against Bobby's crotch. "Only one way to find out..."
The temptation was so very, very strong...but Bobby just couldn't bring himself to take the chance.
"I'm sorry, Katie. I'm not ready to take that chance, at least not yet."
"What?!"
"It's not that I don't want this. I want this. I really, really want this" (and Bobby's erection made it clear that was the truth) "...but it's the possible consequences..."