"Um, well, you see, no. Any chance you could come back for swimming another time? Rose and I were just... uh. Busy. With things." She'd understand, wouldn't she? From what he'd gathered, Donna and Jack had been busy with things of their own, and Jenny had to have some sense of what these sorts of activities meant, right?
Knowing Jack, she'd likely be assuming there were a lot more parties involved. Still.
"Tell you what. I'll take you to the swimming pool planet, eh? Later. A few hours later, hm?"