A sexualized version of "in for a penny, in for a pound." Means almost the same thing as the original saying but in a sexual context as aforementioned. In context, it references sex with or without protection (in this case, without), and since it was done before might as well
double down and do it again regardless of the
consequences.
This was used in Team Four Star's,
Dragon Ball Z Abridged: Episode 60 Epilogue.
Bulma: "But after
the first time, I just
thought; well, in for a penny, in for a
pounding."
Trunks: "Ahhh!"
Bulma: "And I really was."
Trunks: "AHHHHHHHHHHHHHH!"