I could actually see Ami grabbing Usagi and her girlfriend and pulling them back into the hallway, causing Usagi to pull the door closed, then making up some terrible excuse while a frantic shuffling comes from the other side of the door.
Just to clear things up, tvtropes does state that Artoria Pendragon is bisexual from various scenarios that happen in-universe. (Unfortunately these scenarios are pretty much porn though). Not that it makes her stand out; there's more bisexual female characters in Nasu's works than there are straight ones.