Kitt has worded it perfectly.
While my Husband and I do have a big house (4 bedrooms, 3 floors) for two people with no kids....but our stance remains the same.
If we have invited people over, we have the room to put them up. (Our visiting friends usually leave the place cleaner than when it was when they arrived, and they also always bring food and other items, even though they would not need to)...it's a matter of being polite, and respect.
And, out of that same respect, we would state the exact same thing if someone were to either just drop in or make an assumption that "because we have the room" then somehow we "must" be able to put them up. The decision is not theirs to make, and like most people, we, too, have schedules.
Nipping the situation in the bud is ideal if a phone call is being made (with presumptuous agendas), and a very kind, polite, and gentle reply that you'd be more than happy to give them a number of a local B&B or, as Kitt stated, be happy to make the reservation for them.
I may be in the minority, but I do not like unannounced visitors. I merely ask for the courtesy of a phone call in advance. I do not appreciate assumptions being placed on my family and our time, that we will or can stop in our tracks to put things on hold that could just as easily be planned for in advance and work out to everyone's benefit.
It's about courtesy. It's about respect. It's about consideration.
M.