My suggestion for a sign also teaches Catholic values. Audience awareness – in writing, speaking, marketing, etc. – is important. Unless, of course, one just wants to be right instead of persuading others to accept Catholic teaching.
Many Catholics today believe in SS marriage. For example, there is a Roman Catholic college in Worcester, MA, College of the Holy Cross, which publishes a magazine called “Holy Cross”. In the magazine, there is a page called “Milestones” where they congratulate graduates of the college on wedding or other special events. Pictures of the event are published. In the last few issues, they have congratulated SS couples on their marriages.