My co-worker invited me to her wedding to another woman. Do I explain why I'm not going or just decline the invitation? And do I send a gift for something I don't believe is valid? 

This is a very difficult situation, with no standard solution. The assault on the sanctity and even notion of marriage has put faithful Catholics in a very uncomfortable position. Novel, previously unthinkable norms have set us back on our heels, so to speak.

You’re to be commended for your love for the truth and your co-worker. Her invitation suggests that she’s unaware of your beliefs and the dilemma the invite creates. If that’s the case, it’s probably not the time to try to explain your position. She won’t have ears to hear right now. There may come a time later when you can explain your position on this topic.

Be strong, but subtle. Since either attendance or gift-giving is an affirmation of the event, either would compromise your convictions. Though you can’t support her decision, you can still be a friendly, collaborative co-worker.

Your decision probably won’t enhance your profile in the office. You might even be unjustly labeled an intolerant bigot, ironically by the preachers of the gospel of “tolerance.” Take heart. Jesus paved the way and St. Paul reinforces the principle. “Indeed all who desire to live a godly life in Christ Jesus will be persecuted.” (2Tim 3:12) You’re in good company.