crabs, or more accurately pubic lice, are parasitic insects that use a host mammal for food (blood) and as a warm, moist place to incubate their eggs. the tiny lice eggs are called nits, and it is from this we get terms like nit picking, nit wit, etc. the creatures cement the nits to the hairs, near the base, and the way to get them off is to go over them with a fine-toothed comb, another expression left over from a time when personal infestations were far more common.
although adult lice can live quite a long while away from a host, they do not prefer this and will take advantage of any opportunity to find a new host. it is rare to get infected from a hard cold surface like a toilet seat, but it is possible. it's far more likely you got the little buggers from upholstery, blankets, a car seat, clothing or the like.
of course bumping uglies is the best way to get crabs, and 25 years of marriage doesn't really guarantee shit. there is even a rare chance that your wife got them from fucking a canine, and since that seems like it would be the most offensive that's the theory i'm going with.
you got crabs because your ol' lady's been fucking fido.
