If we were never to introduce new food, then we would always want breast milk.

Actually, weaning off suckling on breasts shouldn't be viewed as social conditioning. If you observe Nature, you won't find members of any species suckling mother's milk all their life. It's just an initial arrangement where infants feed on what mothers' are eating and digesting for their babies. But once the baby grows older, she can eat and digest on her own.

How is a meat-eater who cares about afforestation/environment a contradiction in itself?

I agree with your point of crop rotation to keep land fertile and productive. It's not that we are abandoning arable land but because we need more and more land for growing feed for animals, we are destroying forests for our land requirements. Esp. refer the case of destruction of rain forests to cultivate palm trees for palm oil. Please Google up a little and you will find all relevant details regarding this.

there is no animal or pet that someone throughout the world won't eat.

Come to my place some day. You won't see anyone eating animals here.

What others are doing shouldn't be the benchmark of our own activities. We should held ourselves responsible for our own actions.

When a small child stomps on an ant or spider, there is no compassion

I disagree here. I've seen many small children playing with ants and if they get killed by them accidentally or unknowingly, they cry. One of my friends used to rescue flies and bees from water when they landed in swimming pools when she was a small kid. She also used to put stray worms in the middle of the road to safety. A friend's son started playing with snakes when he was still in cradle and his love with snakes continued in his growing up years. Today, he rescues many snakes in his town as a hobby.

But yes, not everyone has the same level of consciousness and their compassion & understanding may vary. But fundamentally we're all the same.