You know there's really not much XX happening to girls from different species.
The japanese title uses OO (marumaru) instead of XX.
As far as I'm aware the expression OO would more or less translate to "blank". Meaning that the title would translate to "A Story About Doing blank to Girls From Different Species".
I believe translating it as XX makes it look more sexual than it actually is.
Then again I'm neither a native english speaker nor a native japanese speaker. So there is a good chance that the scanlation group knows better than me.
There's really not a standard English for a blank, undefined word outside of a blank underline, and that would look silly in a title. As would (blank).