This path is available in the help section of the game. On the main menu click “Help” then click “Paths” on the top row, and next to where it says “Hot Tub Etiquette” click “Enable”, then load up a save of pizza dinner or steak dinner. Correct choices will be highlighted in Magenta. Note: Where there is more than one choice, you might have to do one, then the other when you get to the choice page again. If you did not save an opening, enable “Pizza Dinner” or “Steak Dinner” as well and start a new game from the beginning.

Start with Pizza Dinner or Steak Dinner:

Go to the bedroom to put on a swimsuit
Wait for her in the hallway
Stay in the backyard
Get in the hot tub
Suggest playing “Truth or Dare”
Truth  or  Dare (does not matter)
Yes  or  No or Finish dare (does not matter)
(Pick a question, does not matter)
(wait for her to finish answering)
Dare
Dare her to take her top off
Dare
Dare her to get naked
Kiss her
Kiss her nipples
Massage her thighs
Grab her butt
Rub her pussy

And that is how you have sex in the hot tub