A bakery owned by a Christian family asked Britain's Supreme Court on Tuesday to overturn a ruling that it discriminated against a gay customer for refusing to make a cake supporting same-sex marriage.

Ashers Baking Co. in Northern Ireland refused in 2014 to make a cake iced with the "Sesame Street" characters Bert and Ernie and the slogan "Support Gay Marriage."

The owners argued they were happy to bake goods for anyone, but could not put messages on their products at odds with their Christian beliefs.

After the customer filed a lawsuit that received backing from Northern Ireland's Equalities Commission, lower courts ruled that the bakery's refusal was discriminatory.

Judges from the London-based Supreme Court heard the bakery's appeal at a special sitting in Belfast that is due to continue Wednesday.

David Scoffield, lawyer for the bakery's owners, argued Tuesday that the family should not be compelled to create a product "to which they have a genuine objection in conscience."