Your analogy is completely wrong. Of course a pet store owner would be within their right to refuse the sale of an animal to a person that's going to abuse it.
A better example would be a shop worker refusing serve a gay man because the shop worker's religion says that homosexuality is a sin.
Religious Freedom is about the freedom to practice your religion, not to use it as an excuse to be an asshole to people.
Unfortunately, that's exactly what religion is about these days.