Had a few more ideas and starts with Glastrier, Spectrier and Calyrex.
I read on Bulbapedia that when Calyrex rides either of them that they each get a different boost. For Glastrier she gets a endurance boost to run for several days so this can turn to she can have sex continuously for several days without getting tired. For Spectrier she gets a speed boost to run a thousand miles in one day so she is able to have sex super fast. Now what would happen if the MC gets to have sex with the three of them at same time. It would likely last for several days and their speed would make the MC cum so much a little bit of a cum waterfall forms during it.
Another idea is that one of the future girls that he needs to show the legend to is the very first girl he ever had sex with in his life but was so long ago that both forgot but not their body's. She wanted him to wear a condom that first time but after having it raw she regrets having missed out on this for so long.