Ah, that was always the terror I had when I was making Homeward... Poor Scout had to endure so much of my questions on the IRC channel because I needed to write about female masturbation and I obviously had absolutely no experience with it! XD I really did not want to write something stupid and get laughed at later, so I had to make sure everything was realistically probable!Well, there's a difference when dealing with sex as opposed to ghosts and mecha - that being that you CAN do these things in the real world, and people who don't have the faintest idea how to do them will sometimes take your writing as an instruction manual. That can be problematic if you were just making it up.
Now that we've moved out of LHH I can't provide examples, but back when I used to review hentai fanfiction I would occasionally come across people who were EXTREMELY CONFUSED about anatomy. I really hope someone sat them down and explained a few things before they ever got romantic partners.
Sometimes though, I really wonder what the big deal is about "writing what you know." I mean, you are making a story. I don't expect stories to contain fully real life accurate descriptions of many things because it'd be too dull. Fiction embellishes certain aspects of reality and cuts out the boring and irrelevant bits to make a compelling narrative.
Especially with the younger members!