"I was bitten once and it was highly unpleasant," he said bluntly. "And I have a mate."
Cas wasn't especially good at reading others and decoding what it was they meant when speaking, but he knew that a female offering to strip down was often an invitation. Rather than be accusatory or start an unwanted argument, he felt it was best to simply inform her of the truth. He didn't know what else to do, so he relied on what had been useful in the past.
"Nets...would be more useful," he agreed. "But for the time being, I would much prefer to commit an act of violence. While I draw no pleasure from taking the lives of the fish or any other animal, they will be contributing to a certain cause."