(AiE Fic. Fair warning if that's not your cup of tea.)
Anon and Dash have a nice relationship going, according to them.

Not really a couple, they decided to have a 'friends with benefits' type of friendship. Neither of the two are interested in settling down, but urges don't take care of themselves. They like the idea even more since Anon is incapable of impregnating any of the ponies.

Feeling that their sex life is getting dull one day, Anon and Dash head to the library for ideas, much to Twilight's dismay.

What they find might just change many things for them.