In the UK and the USA a lot of people feel a sense of disgust and abjection towards feet. Of course, in many Asian countries, the foot is considered to be unclean. However, this is in connection with the fact that the foot touches the ground. Thus, shoes are considered to be far more unclean than the naked foot. In an Asian home, barefeet are acceptable, but shoes are not. This is actually the very opposite of the western abjection of the foot.
It is very common in internet discussions about shoes-off in homes for the subject of barefeet to be raised. It is argued that barefeet are disgusting, more so than the dirt on peoples' shoes. Of course, if you do feel that feet are disgusting, you can still ask visitors to remove their shoes if you lend then flip flops or socks to wear. Angie mentioned this in a previous post.
It is very likely that the sense of disgust about barefeet will decline. Sandals and flip flops have become incredibly popular in the UK and the USA. People are becoming more used to exposed feet. And ladies (and maybe some men) are spending good money on keeping them looking nice.
The argument that feet are more unhygienic than shoes is quite wrong. Unless a person has been going barefoot outdoors, they will not have been picking up the awful things that the soles of shoes pick up (though sandal-shod feet do get a bit dusty). You may think your feet are disgusting, however, you undoubtedly have more germs on your hands than on your feet. Feet are usually remarkably cleaner than the average pair of hands.