It was a mistake allowing Faunus to bring his centaur friend Appleton over. The second Willa saw him, she fell in love. She was, like always, nursing the triplets, and Appleton just walked into the room. Their eyes met, and they knew they were meant to be together.
Appleton and Willa started their romance slowly. After all, Willa's body was still recovering from the birth and she was currently nursing. But when the triplets were a year old, they couldn't help themselves any longer.
During the triplets' first birthday party, Willa and Appleton snuck away to have some private time in the woods. It started with intense making out, then moved to Appleton sucking on Willa's milk-filled breasts. Until finally, they did it. Willa climbed onto a stump and laid on her back, her legs spread wide, allowing Appleton to plunge his massive horse cock deep into her. She moaned and cried with ecstasy. This was so much better than the conception of the triplets!
But alas, it was the only chance they'd have to consumate their relationship for a while, as Appleton had to travel for work. Willa, for her part, went back to being the triplets' wet nurse. They were starting to get very attached to her, however, which would make leaving them to live with Appleton difficult.
A couple months after her encounter with Appleton, Willa noticed troubling symptoms. Ones she remembered from her pregnancy with the triplets. She snuck out to see a witch, who confirmed her suspicions.
She was pregnant. Again.