Um, NO, lol, they certainly are NOT monogamous!!! Huge breeding operations, like the kind that sell their baby Budgies to the big pet shops, have massive breeding warehouses where they set-up tons and tons of nest-boxes along the top of the walls, and they have hundreds and hundreds of females, and half as many males...And you can actually watch videos of the male Budgies just going from nest-box to nest-box...So what is happening with your Budgies is completely normal...HOWEVER...
What you are doing is called "Community Breeding", and I highly suggest that you stop it after these clutches, because inevitably when you community-breed, not only are you bound to have a ton of inbreeding happening with Budgies, but also you are almost assured to come home to a complete bloody massacre one day, where your loving, friendly "colony" of Budgies suddenly started attacking each other, usually a female will try to get into another female's nest-box, and you find both females dead, multiple chicks dead with their heads decapitated, limbs all over the place, they'll be ripped-open and all over the cage, and it's just awful...This eventually happens to EVERYONE who decides to practice Community-Breeding of Budgies...And I speak from experience, as I was born into a Budgie-breeding family, and bred both American and English Budgies for over 20 years...No matter how well they get along or how peaceful your Budgies are right now, it doesn't matter...
And just to answer this question ahead of time, yes, this does occasionally happen with Budgies in the wild, but not nearly as often as in captivity, because in the wild they aren't confined to one cage with multiple nest-boxes, their nests are in entirely different areas for the males to go around and do their thing...Inside one single cage with multiple nest-boxes, especially with one male being the father of multiple clutches, the females go crazy with territoriality and jealousy, and one female will inevitably try to get into another female's nest-box to kill her chicks, and they will literally battle until one or both are dead, and usually the chicks get completely ripped-apart in the process...
It's awful, and you don't want to look inside of their cage to find it one day...Community-Breeding in captivity is a really BAD idea.