Even better idea for recharging ki via sex!
If you fuck someone that doesn't have a ki pool, you get back 1d4 ki points when they cum from your actions. This makes it so you have to pleasure your partner and cannot just masturbate your way into more ki. Additionally, if both love partners have a ki pool, something else happens entirely. Instead of getting 2d4 ki back, each person leeches 2d4 ki from the other person as soon as one or both of them cums.
So, if one or both of you already have a full ki pool you'll take the difference from your individual rolls and go from there. Each has 10 ki, the girl rolls 5 while the guy rolls 6. The guy would still be at full while the girl would be down 1 ki point since the drain happens at the same time. If you have zero ki then you cannot lose and would get ki while your love partner would get nothing.
Adds a bit of fun randomness with ninja on ninja action.