Many parrot species, like a lot of songbirds will mate for life. My grandmother always said
not to kill the mourning doves because they are mated for life.
I have a pair of congo greys that I have had since 1990. During the 1990's the male couldn't fertilize his mate's eggs so I "borrowed" a male from another member of the Rocky Mtn Society of Aviculture, for about a year. The hen never bonded with him and never laid eggs. I did keep the first male within hearing distance so that might have had a part in her rejection. When I returned the borrowed male and put the original pair back together, they were so happy & she immediately laid more infertile eggs!
So they are still together and I don't keep them with a nest box.
I have also seen a male bluebird keep two hens busy in 2 separate bluebird boxes in the wild,
so there is always the exception to the rule!!!!!! So it may be more an innate female preference
to be "faithful"...
If the birds you buy are young, no strong bonds are probably present yet.
Good luck!