If one commits the act of sodomy with a cow, an ewe, or a camel, their urine and their excrements become impure, and even their milk may no longer be consumed. The animal must then be killed and as quickly as possible and burned.
When you have a gown, there's much more to be concerned about. Where is this crease falling? Are you making a weird shape with the dress? Are you doing the designer justice? With a bathing suit, it's more about you and the mood you convey.