AKA: making the penises kiss
A real pet peeve of mine is when fanfic authors don't understand how the concept of gay sex works. You can't just stick your dick inside the guy without any preparation or little to no warning beforehand. Without any preparation or lube (and no, spit does not count but it's better than nothing) it's going to fucking hurt. Here's some imagery to get my point across:
You see that? It's so romantic and full of passion, right? Damn, I wish I was that guy.
((End my existence))
I guess the only real excuse as to not write them preparing themselves is that it's a rape scene, as it's supposed to be a horrifying and painful experience (I have a rant on this subject but I'll save it for another chapter). But if you're going for a lovey dove-y scene where everything is sunshine and rainbows, I would recommend including it in the story. I understand that fanfiction isn't real life, where some things can be looked over without worry, but if you're going for a story that has even just a hint of reality then you would just do it.
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If you thought of Shia Labeouf when you read that last sentence, you need to be stopped.