In her pussy.

"Okay Doe, just a second."

"Hurry John, I'm not on birth control. You aren't even wearing a rubber. I DO NOT WANT TO GET PREGNANT."

You were about to pull out, when it started. In the split second before you cum, nothing can distract you. The Pope could have a gun to your head and you'd still finish cumming. You honestly meant to pull out, but you started cumming faster than you thought you would. You didn't have time to pull out, and you filled Doe's pussy up with your cum.

"Oh, god damn it, GOD DAMN IT. John, what the fuck?! I told you to pull out!"

"Jesus Doe, I'm so sorry. I swear I tried. It all happened so fast..."

"Yeah, I wasn't expecting it that quick either. Fuck, I hope I'm not pregnant."

You didn't have sex for the rest of the trip. Neither of you bothered wearing clothes for the rest of the time you were there, and you did get pretty handsy with each other, even doing some oral. But you didn't fuck any more. You were too worried about pregnancy.

Well, a few weeks later, you got a call. Doe was pregnant. And it was DEFINITELY your baby. Furthermore, she was keeping it. You got married a couple months after you found out, and you raised the kid together. It's not exactly how you hoped things would turn out, but hey. You did keep having amazing sex.