It's not unusual for a pony's name to become associated with an activity: the very nature of Equestrian appellations just about guarantees that anypony who's truly famous for their deeds might wind up in the history books and dictionary. So in that sense, Twilight probably should have expected Ponyville's population to start using the Bearers' names as shorthoof descriptions.

She just wasn't expecting anypony to be using them to describe sex.
She'd also really like them to stop.
And she's just a little curious about which sexual acts are involved...

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Story title suggested by Spamotron: chapter title by Goldfinch142