I don't allow it, the taking up of two seats, I just move their bags onto their laps and sit down.
Re the first class, I do know that sometimes people will buy up a whole compartment and have 'reserved' stickers on the windows, so that they can travel in peace.
I too hate the legs invading my space, but I do sympathise with them. Wouldn't we all like to be able to spread ourselves out - but we can't. However, the seats are at the right height for my legs, not for those men with much longer legs. It's like me sitting on a child's tricycle.
The worst example of manspreading was when we booked a whole table for us so that we could travel back from Newcastle to London with a 5 year old and an eight year old at 6pm. Two businessmen were on our table, clearly marked with our names and they were removing them! They refused to give us our table and seats. As we had just passed the guard as we moved down the train in search of our carriage, I stood there with the children, glaring at them and repeatedly asking them to give us back our seats while DBH went to fetch the guard, who took one look at the tired children and forced the men to go elsewhere. It made me wonder how many other people suffer from such rudeness. Why on earth hadn't they booked?