Lodging is really a short term stop gap sort of thing, or week days only for those who live too far to commute to work.
It isn't the way many people live for years on end - except for those who genuinely are happy to go to work, come home, sit in their room with a microwaved meal all night and repeat day in, day out (they do exist, but they're not very common)
Very normal to have just your own bedroom with bed, desk, chair in it - use of the kitchen is normally a quick in and out, timed so as not to interfer with the home-owner.
It is not normal to assume, as a lodger, that you can sit around in the kitchen, taking your time or making complicated meals if the home-owner is also home/due home.
It is an easy deal for someone who doesn't mind their own company in the evenings, has plenty to do out of the house during the day, possibly isn't around at all at weekends, and wants a very cheap place to live.
Its not at all suitable for someone who wants communal living in a shared house/flat, where shared rooms are on a 'first come wins use of the room for as long as they want it' basis.
Does sound like OP's lodger has told, or allowed his girlfriend to think this is a house share and he is an equal house-mate.