The truth from both sides

You make sure your dad is not on a business call before knocking. "Dad, I have something important to tell you, but I am not sure how."

"You are having sex with your mother and she is a mermaid." You dad guesses bluntly.

"Yes, how did you know." you reply shocked.

"Your wet hair, embarrassed tone of voice; I saw the bath salts on the side as the bath was running; and I agreed it was about time with your mother last night. We want another child you see but before lockdown we got tested. Your mum's as healthy as she has ever been, at least where it counts. My sperm is practical dead there is only a 10% chance of us getting pregnant from me. You boys are the next best thing as your swimmers are still healthy and genetically half me. As for the mermaid bit. Your mum has been trying to find a way to let you know for years. When Lockdown came into effect, we decided to kill two birds with one stone and should you find out about her true form she should encourage you to give her a child." Your farther concluded.

"So you are fine with us having sex." You concluded.

"Knock your self out or more accurately knock your mother up." Your farther joked.