Hannah also wants you to stay.

As soon as Hannah gets out of the water she grabs your other arm and says to you, "John, we just met you! You have to stay and hang out with us!"

While she says this she is clearly looking down at your dick. Having two girls touching you while you are naked and knowing that they are looking at your dick makes you start to feel like you are going to get hard.

"Come on, John", said Kelly, "let's go lie down and talk for a bit!"