Trying to keep an unaltered female in heat from an unaltered male would be like trying to take a relaxing nap in a heavy metal concert. Everyone concerned would be absolutely misrable during each heat cycle. Since you mentioned having the female spayed and not the male neutered, if you are concerned about not having testicles, you could check into Neuticles - here is the link to their homesite
Neuticles.com. A comment directly from their site:
Neuticles allows your pet to retain his natural look, self esteem and aids in the trauma associated with neutering.