I am wondering if it is allowable for them to have built the floor length windows overlooking your house - do they also overlook the garden?
I would get some really fast growing leylandi or bamboo to put on your boundary, not necessarilt the whole length but strategically placed to block their window and keep it cut at a point that only allows you to see the upper part of their window. Alternatively perhaps a cafe blind in your kitchen window would do the job if you still have privacy in the garden. If you can see them, they can see you depending on the sun and lighting.
Or you could put some anti itch powder through the door with a note saying “ I see your husband has to scratch himself every morning perhaps this will help”, but it sounds as though they know they can be seen and don’t care - perhaps they are naturists.
Another explanation is that they have the type of window that is supposedly non see through from the outside. One of my neighbours has these with no curtains, but if there is more light inside than outside, ( or if the sun is shining into the room) you can see in very clearly.