Just skipping past the difference between ass, and arse. It's words in context. One of the things that puts me off of a story is breaking my suspension of disbelief. So, using your example, a british exchange student saying "Arse, Biscuits," or "Crisps" helps me suspend my disbelief.
These dialectic differences help tell people's speech apart in dialog. You don't have to keep repeating "She said," or "The British exchange student said" if you can tell the difference between How they speak. Likewise, there's feminine, and masculine ways of speaking, and thinking (From cultural roles) that we pick up on, subconsciously, because we're immersed in the dialog.
What does take me out of the immersion are things like a bad simile, repetition, unnecessary description, Veronica Marsing (The opposite of Buffy-speak. High schoolers talking like graduate professors of philosophy, because they're not written by high school students, even valedictorians.) All things I use as literary techniques when I want to remind the reader they're reading a story, but try to avoid when I want to immerse them in a conversation/sexual action.
Context, how and when the words are used. By extension, when you want to say Hardon, and when you want to say throbbing turgid manhood. In this example, you want her to gasp at the sight of his throbbing turgid manhood, but when it gets down to the meat-piston action, you want to focus on the action.
Also, realistically, people just say fucked up shit in bed. After they collapse gasping, and sweaty in a bedroom steamy with the scents of lust, they might catch their breath, and do pillow talk, but after a fast exciting break of "Ooh, ah, yes. Faster, faster!" Did I leave the stove on?
At this point, words fail them. "How was that?"
"Great." What else is she going to do, describe the sensation down to the nerve ending, or just absently play with the puddle of semen on her panting midriff?