So why don't you speak up? Say 'Oy! I was here first', or 'MY sofa actually!', or 'you're welcome to sit beside me but I do not want your legs on me. There's a whole spare sofa over there'. The sort of everyday assertiveness that can be good-natured, or irritable, depending on the tone of the behaviour and feelings involved.
This is what would happen in a normal relationship.
You would - good-naturedly or irritably - assert your own right to sit on the sofa where you were first.
It would probably begin good-naturedly (Oi, I'm sitting here! Move your legs!) and progress to irritable if the person was refusing to listen. Culminating in a row, if need be.
Because it's selfish, entitled power-play behaviour that has no place in a happy relationship or co-habiting situation.
The question is, why do you not speak up? What do you think would happen if you did? Why does it not even occur to you that you could?
You haven't answered if you've ever challenged this, or if you have always accepted it.
Now that you are getting used to the idea that it is not normal, what now?
Will you try challenging it? Or is it better to find some external help (counselling, perhaps) to work through what's going on with your boundaries and behaviours? Just counselling for you, though, and keep it private. You need to explore with someone's help what to do next. Don't tell your husband.