Now we’re getting into the jealousy territory. And it’s important to consider two things here with Usagi’s jealousy. As she notes here, they have literally just started going out, in both the anime and manga and haven’t had the oppurtunity to be together much between fighting evil and Mamoru being brainwashed. In the manga this comes pretty shortly after the Usagi debrainwashed Mamoru/killed him/killed herself/saved the world, two weeks later maybe, a month tops, and in the anime it’s veeery shortly after she finally got him to remember they were a couple.

The point is, they haven’t had much time to adjust as a couple and be comfortable with each other, and suddenly this kid is thrown into the mix and everything changes.

Another thing to consider is that Usagi is used to having Mamoru entirely focused on her. Like, Mamoru does not have much in his life outside of Usagi. He has no relatives, and unlike Usagi, no close friends. He is just completely and totally devoted to her. And that’s really flattering, and maybe Usagi was starting to get used to it, and then all of the sudden his attention is mostly off of her and he’s showing the same level of devotion to this random little kid they barely know. That’s got to be pretty confusing, disorienting and kind of a bummer.