Pandas are a female-only species. They must mate outside of their own species in order to survive. Female Pandas can mate with any male of any species. However, 90% of the time they choose male foxes and most of the remaining 10% are other male canines (wolves, coyotes). Male foxes and female pandas have gotten along the best over the past 100 or so years. Not only that, but vixens are the most tolerable towards the pandas among the canine species.
As you all know, the male fox anatomy is very different from that of a male panda would be (that is IF there were male pandas*). The male fox can swell up to about the size of a baseball or slightly larger. Which could potentially cause pain but the female panda body has evolved to mimic the muscular walls of the average vixen. Fox semen doesn't need to make any changes to accomodate the panda.
After about 4 months the panda gives birth to 2 to 5 young. At the time of the birth, almost all fox/panda couples are married. Usually the couple will produce 3 or 4 litters in their lifetime.