Some places don't mind mares adn geldign mixed but most yards don't allow this.
The reason being is mares come into season, start winking at geldings and get rather difficult to handle. geldings can still get the instinct to mount a mare and this can cause injuries. To the mare especially if the gelding wears shoes and to the gelding should the mare kick out.
A gelding may be a rig that has stayed quiet for a time then put with a mare and
a little bambino arrives.
However a gelding does not need to be a rig to mount a mare.
At one yard the YO dunno why.. decided to put a mare in with the geldings and she was a right hussy. I literally watched her back up so fast and hard into my lad that he must have thought heck, either mount her or fall over.. So yep he mounted her and did the business for about 2 seconds.
Now my lad is 22 years old and had for 8 years been turned out with 35 other horses that were mixed mares and geldings and no little ones emerged so I know he is not a rig.
Also geldings may fight over a mare, again not because they are rigs but because of natural instinct.
To save injuries and disarray its easier to keep mares and geldings seperated