"They know you wouldn't mind their relationship, but they still want to keep it a secret, at least for now. It's just kinkier that they do it behing your back and that you have no idea what's going on right under your nose."
"The whole family urges them to hurry up. After all they'll have all the time in the world to fuck after the wedding. But they insist on doing it right before the ceremony, so that your wife would take the marriage vows with her pussy full of your dad's fresh cum."