Yes, some butterflies are territorial. They will "patrol" patches of plants that are good nectar sources so they can mate with the females when they come in to feed. Monarchs don't really patrol. They don't use anything special to determine whether it's a female monarch or not, so they will attempt to mate with anything that looks similar to a monarch. Once they realize things aren't quite working, whether it's due to the fact that the other butterfly isn't a monarch or isn't a female, they give up.