I 100% agree with the previous posters! To add, you may find that your mare turns into a huge "hussy" or that your gelding get a bit overprotective of the mare, but both of those things can be solved relatively easily. There's certainly no for-sure reason not to pasture them together!
I know my mare can't be pastured with geldings because she starts herding them around, cornering them to lick them, etc and she gets unbearably herd-bound to her "gelding of choice." So I choose to not have her around geldings because I don't want to deal with it [among other reasons]. But, if I were to take the time, I'm sure she'd get over it.
My mare's also the only mare I've ever seen behave like she does. I've been around plenty of mixed gender herds without witnessing nearly the level of drama my girl creates over a single gelding.
So you're probably safe. :)