He couldn't believe that the first thing, she could think of was going to the zoo. He thought that she would ask for something else, but not heading to the zoo. Since he had promised her to do anything she wanted, he had no choice other than to accept going to the zoo the following day. Cat noticed that he wasn't happy with her choice of heading to the zoo. But she was pretty determined to head to the zoo while on her honeymoon. It as always been her dream to visit a zoo when she was younger. That's why she took the opportunity when Shawn informed her to say what she wanted.
"Alright my wife we will head to the zoo", even though he didn't want to head to the zoo. He had to take her there since Cat wanted to go. He promised her to make her happy, that's why he decided to take her anyway.