I use "they" for a single Person / multiple Persons - their Sex not specified.
I also use "them" for a single Person / multiple Persons - their Sex not specified.
Then you can use "their" for properties of single Person / multiple Persons.
It also covers trans-gender People, or maybe Aliens who have NO Sexual bias.
And of course Aliens who may have Three or more Sexes.
( See Azimov - "The Gods Themselves" - three Sexes, which eventually fuse into one ).
I also call all Sentient Beings "People" and where there may be a number of unspecified Nations, Races, etc. - I use "Peoples".
I pnce had to write some Rules for a Group which included Males and Females - so I hit upon "they" which is either singular or plural - not specified. But which does not imply any Sexual bias.
Also Person, Persons, People, Peoples - do not imply any Sexual bias.