Glynda can't have children
Many years ago she went on a mission to clean Grimm out of a city when they invaded, She was with Qrow, Summer, Ozpin, Port, and Oobleck. They had to be scattered around the city to even have a chance to save it, Qrow and Glynda were clearing out children when a Beowolf was about to kill two of them, Glynda saw and moved them out of the way, dropping her weapon on the way. She was wrestling with it before becoming tired and getting hurt. Qrow screamed and shot it off of her, seeing her with three claw marks right across her stomach. She was bleeding out; the rest of the Grimm were killed and Summer rushed over trying to calm Glynda down while Qrow was hysterical and Ozpin was in shock. Oobleck and Port got help but it was too late, by the time she got help and was rushed to the hospital they told her that her reproductive organs were damaged beyond repair. Qrow had blamed himself ever since, that's why Ozpin also convinced her to become a teacher; to make sure she couldn't go on anymore too dangerous missions and play with death again. She still has the scars.