A “sexual joke” can be very hurtful to other people.
Depending on the nature of the joke, and OP’s involvement it may or may not be an issue. It may point to some other aspects of OP’s personality or character that may not be in line with the school’s desired image of an “inclusive community where all people feel safe, supported, welcomed and celebrated.”
AO’s aren’t just looking for people that will positively contribute to their community. They don’t want to have creepy students around anymore than they want automatons.
What is iffy behavior when living at home can blossom into full on BAD once away at boarding school. Like, joking about sexual assault can devolve to actual sexual assault. And yes, BAD things can happen at boarding school.
And just because you are joking in Snapchat (or whatever the kids are using now) doesn’t mean that there isn’t a frenemy screenshotting. The older sibling of one of my kid’s former classmates found out about that the hard way - one really stupid post on Snapchat that someone screenshotted and forwarded, and the college rescinded their acceptance and athletic scholarship.
Again, lacking details, all any of us can say is, “it depends”. None of us know what OP’s school’s official end of year transcript includes.