Hi, Limeytea. I can’t say if it constitutes sexual assault, but it is definitely a violation of your boundaries.
“Sexual assault” is pretty hefty accusation (the kind that can ruin lives), and so personally I shy away from the label. If I look back on my life I can remember plenty of awkward social encounters and flirting that could be interpreted as sexual assault/harassment (both the other people and me being considered the “harrasser.”) I don’t personally consider myself to have ever experienced “sexual assault” even though there have been several people in my past who I have felt exceptionally uncomfortable around. There have been people who didn’t take my “no” seriously or considered it a joke to ignore it.
The reason I don’t consider it sexual assault though is because when I look back at those times in my life, I had not yet developed the skill of defining and enforcing my boundaries. I was uncomfortable, yes, and many times, I even said something along the lines of “stop it.” However, I always delivered it in kind of a wishy-washy way… The trailing, whiny “staaaaph…” The leaning away or avoiding eye contact and just hoping that they’d pick up on the fact that I didn’t like it and stop. I was afraid of hurting others’ feelings or getting into a fight, so I subconsciously decided to be uncomfortable instead of risking conflict. The thing is though, I can’t really fault someone for not being able to read my mind…
Should it be enough to say “please don’t do that?” Yes, absolutely. In reality though, it’s often not enough. I obviously don’t know how you tried to define and enforce your boundaries with this person. Did they do something wrong by ignoring your protests? Absolutely, yes. They were wrong. If it were me though, I’d consider how clear I was in saying “no” before I started seeing myself as a victim. Your boundaries were violated, and that does feel gross and uncomfortable. However, there is absolutely nothing you can do to change what happened in the past. What you can do is consider how you could better define and enforce your boundaries next time someone encroaches upon them.
If this is someone you have to live with, yeah, it does change things a bit. You said it happened as an adult and as a minor… Hopefully you don’t have to live with them anymore now that you are an adult. Leaving is, hands down, the strongest way to enforce your physical boundaries. When you were a child, you likely didn’t have that option.
I can understand completely how someone touching your butt would make you feel uncomfortable. Just the idea of anyone other than my partner touching my butt makes me uncomfortable. I had a group of online friends a few years ago who made a big joke about butt touches when they’d meet up at conventions… I never went to a convention or met them in person, but when they’d start joking about wanting to meet me and how they’d give me “a butt touch,” I felt super uncomfortable. At first I treated it somewhat jokingly, saying things a bit passively like: “Please don’t touch my butt! 
” Note the emoji. I meant to kind of soften/avoid the conflict, but in reality it just diluted the seriousness of my protest. The butt touching jokes continued and even escalated for a while until finally one day, it was enough. I was fed up.
I was streaming live on Twitch when one of these friends started joking in chat about how they wanted to meet me at a convention so they could touch my butt. I stopped playing my game, went to full screen camera, looked dead straight into the camera and addressed this friend by name.
“[Friend], I would absolutely love to meet you in person one day. If we ever do meet, I would love to give you a hug, but I do not want you or anyone else touching my butt. If you attempt to touch my butt, I will physically jump away to avoid the contact, then I’ll tell you one more time that I am serious when I say I don’t want you to touch my butt. If you try again to touch my butt, then I will simply leave you where you stand, and I will not spend any more time with you. I do NOT want you touching me like that, and I do NOT want to even joke about butt touching. If you make one more joke about touching my butt, I will ban you from my channel and block you on other social media sites.”
It worked! I had very clearly described my boundaries and what the consequences would be for violating those boundaries, and that friend never joked about touching my butt again. It worked so well, that I had that same exact conversation with every single friend who tried to joke about butt touching, and one by one, all of my friends finally got the message and stopped joking about it with me. Funnily enough, I think a lot of them were uncomfortable with this joking because I noticed they stopped joking about it, not only with me, but even with each other.
So this was a very long-winded way for me to say that in my experience, I’ve found I’m best served by using my time and energy to control what is in my power instead of focusing on how others should behave.
so it’s just different i’d say. in my experience, i tried to say that i didn’t want it, but i was overpowered and the person didn’t care what i said. also i was very scared so idk. it can sound sorta like victim blaming when you phrase it like that. not trying to attack or anything, that’s just how it came off to me.