I recently attended two wedding receptions. I was shocked at how some of the people came dressed to both occasions. Some people came in shorts, T-shirts, gym attire, flip-flops and jeans. Isn't a wedding supposed to be a dressed-up occasion? Have people become so lazy that they forget the importance and how special this day is for the bride and groom? The newlyweds just spent a lot of hard-earned money to create that special atmosphere and ambiance for their wedding celebration. When people come dressed like they are just out running errands, it signifies that they just don't care and "dropping in" is just a check off their to-do list for the day.
It ruins the whole setting the wedding party tried so hard to create. If the guests can't pay their respects and at least throw on a skirt or slacks for men, why bother? What you wear to a wedding is not about you. Dressing up in (sometimes) uncomfortable clothes, or even spending a little money to look nice is a way you pay a compliment to the occasion. You are saying, "Yes, this is an important occasion, and I am going to go out of my way (a little) to let the bride and groom and other attendees know that I recognize it's special.
If you were attending a wedding on a ranch and were asked to dress in jeans, then dressing in jeans would be your way of complying with the occasion and honoring the couple. If you are invited to a wedding in a traditional service (when it is at a formal reception center), then honor the couple by attiring yourself appropriately.
By the way, nice looking clothes do not have to be uncomfortable.