As a dog lover and owner myself, although me and the mrs have a big dog a German Shepherd, it wouldn't bother me. I personally wouldn't worry too much about it but the fact you've posted it on here tells me you feel a bit put out.
Some people dont like dogs, but the fact they have stayed at yours before suggests they dont mind them but dont want one themselves and it's their house so their rules, and as I said, it really wouldnt bother me personally.