"You should put on the best version of yourself when you go out in the world because that is a show of respect to the people around you." - Tom Ford
A perfect example of this is at weddings. You'll always find that odd guy who goes to wedding dinners, held in posh hotel restaurants no less, in his turned-up-collar polo tee, slouchy jeans and SAF running-shoes.
I strongly believe in dressing appropriately for the occasion. It is also a form of respect to your hosts if you can pay a little more attention to how you present yourself in front of all their other guests. Try and think, like "You know I'm not a shirt, tie and jacket guy. But I'm gonna do that, just for you, on your big day".
I'm sure your hosts would appreciate that.
If they don't, make a mental note to appear at their house-warming party in your muddy football shorts and boots.