Bad books on writing tell you to ‘WRITE WHAT YOU KNOW’, a solemn and totally false adage that is the reason there exist so many mediocre novels about English professors contemplating adultery.Joe Haldeman
A lot of people write erotica about things they don’t know — one only needs to look at the back catalog of Chuck Tingle (world’s greatest author) for examples. I mean, I’ve never had my balls kicked, but I’ve written stories about ballbusting, and I’d like to think they’re pretty good. (Eventually I’ll publish them and you can see for yourself.) And I’ve certainly never received cunnilingus, though that does appear rather often in the stories I write from a woman’s POV.
So I would say this: don’t write only what you know. Think about something that interests you and do some research on it. Talk to other humans. Read some well-cited books or articles. For sex-related stuff, find some amateur videos and watch them. (Amateur doesn’t necessarily mean poor production values; there’s a lot of amateur stuff on sites like Pornhub that is well-shot and slickly-produced.) Try to see people’s actual reactions to things, and do your best to determine if said reactions are genuine.
For example, I discovered Brother Love, a gentleman who makes ballbusting videos in which he seems to genuinely enjoy what is happening, and having seen most of his back catalog I can say with some certainty that there is little-to-no verbal humiliation or degradation happening. It still hurts, though, and he expresses that. Without having watched Brother Love’s videos, my ballbusting stories would not be what they are today. (I did try to find a good link to one of his videos or a page about him, but I couldn’t find anything that didn’t look dangerous. Search at your own risk.)
Another example: I’ve been pegged before, so I know what it feels like to be fucked in the ass. I don’t know what it feels like to be fucked in the ass with a penis, but I know what it feels like to be the giver in an anal sex situation. I can take those two pieces of knowledge, combine them, and turn them into what I think is a fairly realistic male/male anal sex scene. And although I haven’t given head to a penis-owner before, I have experimented with toys, so I have a general idea of what it’s like to give a blowjob.
Writing is about doing research and then making stuff up to fill in the blanks. Don’t write only what you know; keep learning more and using that to write what you don’t know. It’s more challenging, but it’s also more rewarding in the long run.