Big Mac was bragging that his dick is too big for Silver Spoon. Silver Spoon had challenged him to a bet that her anus can take his full cock up to the base. Big Mac was surprised that she could actually fit his cock, but kept accepting the bet again even knowing he will lose. Now he’s just furiously fucking and cumming into her pussy while her gaping tailhole gushes his previous load, his huge dick inside her cum-filled filly vagina producing loud slurping noises.
 
(thanks Background Pony #370E from comments below for proofreading the description text)

imported from https://derpibooru.org/images/2089551