i've used a cup for years, and my attitude about sterilization is that it's kind of totally optional. The inside of a vagina isn't sterile at all, and as long as you wash your hands before handling the cup you aren't introducing anything into your body when you empty it out that wasn't there ahead of time. They just require a scrubbing between cycles. I imagine sponges are similar: you could soak them overnight in salty or mildly soapy water, rinse them out a few times, and then they're as clean as they need to be.

If bike shorts don't help, i'll be checking out sponges.