People can do what they please with ponies. It doesn't bother me one bit. I enjoy fanfics that enter adult territory (sexual and non-sexual). It can make the story better sometimes or can be horribly written. It is the price you pay.
It is okay if you don't though. Everyone has their own preferences.
Normally there is a warning when it comes to R34 or fanfics so it is an enter at your own risk kind of deal.