While a man has mumps, fertility may not be much of a concern. He's too infectious and feels too lousy to have sex anyway. It's in the months after being ill with mumps that he might try and fail to get a woman pregnant. This is because the mumps virus attacks some of the testicles' sperm-making cells.
This attack can actually stop sperm being produced for between 6 months to one whole year. Usually, after a year, the man's sperm count will go up again, although perhaps not to its previous level. Approximately 7 to 13 % of men who suffer from mumps orchitis will find that their sperm counts go down (notice that this is a relatively small number of men - think how many are unaffected).
Even when this does happen, the sperm count generally doesn't go low enough to cause a major fertility problem. A more common effect of the illness is testicle shrinkage. This will happen to roughly half of all men who contract mumps.
The basic message here is not to panic. If you're unlucky enough to be a man who has mumps or you're in a relationship with a man who's sick with mumps, the chances of the condition affecting your future efforts to have babies are very slim indeed.
You should talk to your doctor if you are concerned, but in the meantime, you'd be better off focusing on applying cold compresses and buying some painkillers.