Equestria’s birth rate was in serious decline until Princess Cadance decreed the creation of Sharing Day a few years ago. There is now one day each year when everyone in Equestria must share their spouse or lover -- no hard feelings and no consequences. It doesn't count as cheating. One day each year, everyone gets a pass.

Chapter 1: Shining Armor x Twilight Sparkle light bondage, free use, incest
Chapter 2: Princess Cadance x Big Macintosh casual sex, implied incest
Chapter 3: Mr. Cake x Pinkie Pie casual semi-public sex, impregnation, age difference
Chapter 4: Sugar Belle x Mrs. Cake x 2 stallions cum swapping, impregnation, tits of improbable size
All chapters contain perfectly accepted and consequence-free infidelity and lots of raunchy anthro pony sex!