He meant that you don't just start masturbating to pornographic pictures of animals from a child's TV show in an instant. It takes seeing sexualized pictures of them to develop an unhealthy attraction to them.
I am sure Rarity would strongly object to being called an "animal", for besides the superficial resemblance to an earth creature of the same name, "ponies" are basically people with a different body shape. Rarity is quite the stunning beauty as well, and with her charming and generous personality, and the sophisticated way in which she carries herself, one can be forgiven for finding her highly desirable.
Your opinion of what an imaginary sentient animal would say is irrelevant. There is a difference between being attracted to an imaginary pony's personality and acting on sexual attraction to that pony.
Hoo wee lookie here, moralizing about human sexuality on the Internet of all places? Good luck with that one bro . Also, so what if I find her attractive and fantasize about her, I'm not breaking any laws or causing bodily harm to anyone, so I am entirely within the rights to my freedom of expression. Your opinion is likewise irrelevant, and just so you know I do act upon my impulses ALL. THE. TIME.