Based this and the other teasers he posted, it seems that Flareon is a virgin and Umbreon coaxes him into getting banged and he winds up losing his virginity to a Sylveon crawling with Pokerus ... I assume Umbreon is aware.

Dafuq u ruin everything....anyway i think they will end up having sex :3

That's hardly a spoiler ... As I said, that's simply my best guess based on the three teaser images he posted (at source); this is the only with with dialog, and the other two pages have a big ass "SAMPLE" on 'em, but they paint a pretty clear idea of what happens.

Also note that the doujin will be 32 pages in length, so there's plenty of detail left unspoiled~