Recently at Mass (local, Ordinary Rite, parish) I noticed a guy walk in wearing a Catholic t-shirt, shorts, and flip-flops walk in and sit down, nearly front and center in the sanctuary. Others were wearing everything from quite casual to a few wearing full suits.
If you were invited to dine with the President of the United States...
If you were invited to dine with the Pope...
If you were invited to dine with God Almighty...
What would you wear?
Would you "come as you are," or would you prepare and wear your "Sunday best?" I think most of us would get dressed up nicely for any of these options - and would not show up in flip-flops.
In the third option, we have that invitation at EVERY Mass we participate in! EVERY time you go to Mass you KNOW - or at least SHOULD know that you are going before the REAL PRESENCE of God Almighty. I know, I've heard the excuses "God accepts me for who I am, not what I wear to Mass," but the point is there is a modicum of respect expected when approaching any world leader and should be even more true for the Leader and Creator of the universe! When we do not take the time to clean up or change into something a bit more dignified - or even deliberately wear such casual clothes it is just plain disrespectful.
The problem is that many have lost that level of respect for God, especially in His House - our place of worship. If He is worthy of our worship, He is worthy of our respect too.
In the image below, from Real Men, Real Style, the encircled attire is most appropriate - and even "Casual" can suffice, but not "Ultra Casual" or "Sloppy." On the other end of the scale, while those would not be inappropriate, "White Tie" or "Black Tie" would not be required (and really not much difference between "Black Tie Optional" and "Semi-Formal or Business").