---I would like to participate in a sexual encounter with her.
"I don't like to talk about what goes on between the sheets."
---I do not enjoy discussing my sex life with others.
"We'll settle this between the sheets."
---We will now engage in sexual intercourse to resolve our current conflict.
This phrase is also part of a game where you take any common sentence or title and add "between the sheets." A popular juvenile activity.
The possibilities are endless.
"The Passion of Christ... between the sheets!"
"The Return of the King... between the sheets!"