Of course, we cannot know for sure why Pony and Cherry tell each other these things. But my feeling is it is because they are not really part of the same social circle. They can tell each other "secrets" and assume that the other person will never tell anyone who matters.
Cherry, for example, will also not have to worry about hanging out with Pony and having Pony know her secrets. It's like a one time thing -- they're meeting, talking, but not having to deal with how things will be in a long term relationship. Pony won't have to worry about what Cherry thinks of him now that she knows this stuff -- they'll essentially never interact again.
because they were both the "odd" one out in their own groups