YABVVU.
Have a read back as if your posts were someone else. You are very defensive about your son, him being nice and his GF also being nice. They probably are. As are you, presumably.
BUT you are going to have to take a BIG step back. He isn't your son first and foremost any more. You are not the most important person in his life any more. His house, home and life are not intimate extensions of your own any more.
No matter how polite they are, how well you get on you are still doing something they have asked you not to do. Instead of visiting them as equals, all 3 of you adults, you are visiting them and, on seeing a chore that needs doing, are overriding their wishes about their own clothes in their own home!
Why? What do you feel and think when you make that decision? That they need help, it would be nice to do that little thing for them because.... what? She can't, he won't, they are incapable, slow, slovenly? What?
Some people are fine about such things - look at the mixed responses you are getting here. But your son and his gf are not. They have told they are not, you are ignoring them, and that is what makes mums into MN MILs!
If you continue to do that about little things, big things, any things you will slowly whittle away that good relationship as they feel the need to build fences, distance between you and your good intentions and their new, adult life together.
So next time you see a little job that needs doing in their home, ignore it! It really isn't up to you to judge that it needs doing, there and then, by you, over their protestations!
If you continue to do that you will, without doubt,