Why is it that consent is always the first (and only?) thing that people have a problem with.

Quick analysis: They actually don't think, they go by their immediate gut feeling. And that gut feeling was formed in the first 20 years of your life both by the parents, your peers, the media and political initiatives ("the zeitgeist"). So the conclusion that it is icky/disgusting (an opinion, and even a learned one) comes first. But we all also learned that nowadays you can't come with the argument "it's disgusting" alone. We are a rational society, not a medieval one. So their mind races to a justification after the fact (basically 3/4 or so of our lives, I would surmise, what we think and how we evaluate our decisions are attempts to rationalize after the fact our first impulses). This is icky and super-super wrong, so basically like rape, so - hey! go with that one, and then the person starts to babble about consent, since that's the point in rape, which we determined from the disgusting/wrong feeling this must be in a backwards search for reasons.

That backwards justification search also explains while they constantly fail to take into account every EVERY other circumstance in which we humans do whatever the fuck we want with animals (eating meat, castration, breeding/rearing practices, circuses, hunting,... so much more stuff). That could only be found in a systematic forwards-testing of different proposed reasons why this is wrong. But that requires so much thinking time, your subconscious has figured out the consent justification after the fact 20 times over in that time. And then the phenomenon starts that people cling to their first stated opinion and will not take any fact, no matter how hilarious their position gets from then on. "Uh dude, but you eat meat" "EATING MEAT IS NECESSARY" "Vegetarians disagree" "WHAT ARE YOU, ANIMAL AND CHILD RAPING SATAN SHITOHEADO?" produces foam at the mouth

Just my quick amateur psychology.