It's happening again.

I started dating her about a year ago. It was wonderful. The sex was absolutely amazing. Over the ensuing months, we've spent more and more time together. She's spent more and more nights together.

About three months ago, we got to the point where she was spending one or two nights at her place, the rest at mine. Now, she's been here every night for three weeks. It's been wonderful on a lot of levels. But the sex has declined significantly. Now, despite seeming to enjoy it, she says "no" about 1/2 the time I initiate. This is not unlike what happened in my two previous relationships. And she never initiates. I just got off the phone with a close male friend of mine, and this happened to him, also.

There's something about actually living together that seems to take the romance and the sex out of a relationship for women. Is this true? Why does it happen?