Exactly as Justaphase says. In this specific example, they believed they were the only two down there, they were assured by Clem that no one else could be there. So they wanted a bit of fun, and it was specifically a Luke and Kathryn thing, he was the only one she was presenting herself for. Anyone else fucking her, even where she is and in the position she's put herself in, is raping her, because she's not consented to it. She may have consented to being fucked, but only by her partner, not by anyone else.