If a shop owner tells a gay customer "I don't believe you about actually wanting [the product / service]", how do we know if the shop owner has legitimate reaso

If a shop owner tells a gay customer "I don't believe you about actually wanting [the product / service]", how do we know if the shop owner has legitimate reason to think this or just doesn't like gay people? I'm not a legal expert, but I'm guessing that there would be need to be clear evidence of harassment from the customer (or something of that sort) in order for the shop owner to be able to refuse service. Otherwise, any asshole could get around discrimination laws by simply claiming they don't think customer the actually wants the product. That would be a terrible situation.