TL;DR: I found some porn where the big plot hole wasn't "they're boinking".
So the backstory, as indicated by the dialogue, was that two women, let's call them A and B, who were friends had plans to go to Europe together. However, B thought that A had slept with her boyfriend (which she may or may not have actually done) and refuses to go with her. So A, not wanting to go to Europe alone and having only a few hours before the plane leaves, has gone to B's brother and is begging him to accompany her, and he's not having any of it, because there's no way he could hide or disguise this from his girlfriend.
As I said, this was porn, so we all know how it ended.
Now the WTF factor to all of this is that when it comes to porn, my suspension of disbelief is a fair bit harder to break than when I'm watching anything else. However, by either by virtue of the porn stars being very good actors or being poor actors who didn't put much effort in, the characters came across as so shallow and promiscuous that their boinking would've been completely believable in a non-pornographic context if you edited out the naughty bits. In fact, it was the fact that the guy was ready to leave for Europe in a couple hours on a whim that nearly broke my suspension of disbelief in the end! (though to be fair, it started in the middle of their conversation, so it might have been established off-screen that the guy was on vacation, laid off or otherwise free from work duties)