Characters in order: Apple Blossom (oc), Ambrosia Apple (oc), Cheerilee, Sour Apple (oc), Big Mac, Apple Butter (oc), Sugar Belle, Honeycrisp Apple (oc), and Fragrant Apple (oc)
—-
I didn't like when the show introduced Sugar Belle as a partner for Big Mac, not after having so many background scenes/moments showing the bond between him and Cheerilee, but I do like Sugar Belle, so I figured, why not both? Big Mac is a big boy with a big heart, enough for both of his purple wives.
So the story goes that he and Cheerilee did become close after the love poison incident, though not romantic right away, they simply enjoyed each other as friends and had a lot in common, what with having important jobs in their community that took a lot of time, and both loved young colts and fillies, Big Mac sharing his love for his youngest sister and enjoying hearing Cheerilees take on the CMCs adventures.
But again, they had important jobs that took a lot of time, so when they did become romantic they still kept it fairly casual, and a lot of ponies didn't know if they were still just friends who joked about being closer or actually were that close (hint: they were). Big Mac and Cheerilee had a deal to not worry too much about being physically close or going on dates all the time, and if they had flings with others it was okay too, so long as they remained emotionally close and open to each other.
Enter Sugar Belle, closer to AJs age than Big Mac and Cheerilees, the younger, sugar-sweet pony caught Big Macs eye when Our Town requested supplies from Ponyville, a deal made by Twilight when they opened up after running Starlight out, and Big Mac was one of the ponies to pull a wagon there, bringing them new foods and tools. Sugar Belle picked up supplies from his cart, along with new cook books, loaded of course with mostly apple cuisine, and the two felt a spark.
From there we get the show canon of Big Mac making regular trips just to see Sugar Belle and confessing his feelings, which was a lot harder since he and Cheerilee did have the outside influence to become better friends and then just fell into love, it seemed so much easier than just talking to this new pony from another town who he didn't know so well. But we all know how that turned out in the end, love prevailed!
Big Mac didn't hide that he already had another girlfriend from Sugar Belle, only waiting until their second date to bring it up, and fully expected her to get angry and storm off and never talk to him again, but was hapilly surpised when Sugar Belle in turn opened up about having a poly past already, with her friends from Our Town; Night Glider, Double Diamond, and Party Favour. Her three friends were still together in fact, but she had left their herd after Starlight had been driven out, in order to better find herself again, and was open to being part of a new herd, wether as just Big Macs partner or as both of theirs.
Big Mac kept Cheerilee filled in on how things were going with Sugar Belle, and Cheerilee was glad for his happiness and excited to meet their new special friend, they didn't have to wait long since Sugar Belle started to visit Ponyville regularly, to see Big Mac and even out the travelling between them. The two mares got to meet and were fast friends. Cheerilee loved being able to help Sugar Belle with her self rediscovery and the adjustments she was making in her life, and Sugar Belle got a slightly older mare to confide in and learn from. The fact that both of them loved Big Mac was an added bonus.
In addition to baking her sweets to sell at The Cakes store, them getting a portion of the proceeds, Sugar Belle also learnt her two partners trades, and would help out on the farm and babysit foals. When the time came that they decided to grow their family Sugar Belle would become a substitute teacher when Cheerilee was too ill to work, and Cheerilee would help Sugar Belle with her activities in turn, making their family possible without intruding too much on their professional lives.
Their births were mostly spread evenly between them, with one having a child then the other, with the exeption of their youngest two, when Cheerille had two foals before Sugar Belle had her last. Their family is big, but thats nothing new to apples, and they all love each other very much: if one called a whole stampede would come a running!