I don't think you could see one on a toilet seat. You can get crabs without cheating, though. It's about the only std that you can get from trying on swimsuits at a store, sitting on infested furnishings, etc. I'm not trying to put thoughts in your head, but is it possible he cheated on you and is trying to accuse you of infidelity before you can accuse him?
You cant see crabs out in the open.
http://www.cdc.gov/ncidod/dpd/parasites/lice/factsht_pubic_lice.htm
your gyn would have definitly picked up on it. and no i dont think you can see crabs it could be nats or just bugs. or even fleas or lice not that you have lice or anything its just an example. hope i helped.