Why do women loose interest in lovemaking?

It seems like nearly every relationship I have been in, the woman seems to be content with not having after 6 months or so. Do they just get comfortable and ume when they finally get a guy to stick around for awhile they start taking thing for granted and loose interest in part of their relationship? Why can't they understand that being physical is a very important part of a relationship for a guy and we shouldn't be chastised and accused of "just wanting all the time"? The ladies seem to be interested, too...for a little while.