A gentleman will always give a lady preference when it comes to seating. Even if he had the seat first, he will rise and offer it to her.
Ladies, you are not required to take the seat, but don’t play “tough” by insisting he have the better chair. He’s showing you respect and honor by offering the chair, and you are showing that you appreciate the gesture by accepting it. But, whether you choose to take the seat or not, please be a lady and thank him kindly.
Note: In ordinary circumstances, it can be very awkward for a gentleman to be the recipient of such an offer from a lady. The same goes for getting the door, and other such gestures. Obviously, there are times when it would be very appropriate (he is injured/ill/exhausted/specifically needs your seat for some reason; his hands are full, and he cannot easily get the door himself; etc.), but part of being a lady is allowing him to be a gentleman.