Britain’s highest court on Wednesday said a Northern Irish bakery’s refusal to make a cake bearing a pro-gay slogan was not discriminatory in a ruling condemned by the customer, a gay rights activist, but hailed by the province’s main conservative party. Ashers Baking in Belfast was found guilty of discrimination in 2015 for refusing to make a cake for a customer iced with the words “Support Gay Marriage” because of the owners’ Christian beliefs. It failed in an appeal to the local courts in 2016; but the Supreme Court, the UK’s highest judicial body, overturned that decision on Wednesday, saying...