Realistically, I think anywhere between 1-3x/wk is probably normal and okay, but it really is a question of what else is going on in your world that week.
Hubby and I would love to have it 3-4x/wk but that hasn't happened since the second trimester! LOL But when we were moving, and doing all the packing and planning and everything else, I think we went nearly a month or so without, because we were soooo tired and every waking moment was consumed with things we had to do.
I have never agreed with the idea that a man's sexual appetite was my responsibility. No one ever tells a guy that if his wife wants sex more often, he'd better put out and keep her happy. Sex shouldn't be an obligation. If a wife or a husband wants sexual activity more often than the partner, well, there is a lock on the bathroom door for that.
Now, that being said, I do agree with the idea that sex is one of those things where the more you do it, the more you want it, and the less you do it, the less you think about it. And I also agree that in a good marriage you make time for each other as people and leave the parents out of the picture for a bit. Sex is a good way to do that. I think what happens though, is that men start focusing on the sex and forget about all the things they could be doing to get a woman in the mood to have sex. Romance, a little emotional attention, talking about what he loves about her, or reminiscing about things/conversations/activities/etc that happened when they were falling in love... these are good ways to put a woman in a mood to feel loved and become sexually intrigued.
When it's just about getting laid, it's not as high on the priority list. But when it's an emotional re-connection that is expressed physically, I think both parties get more into it and then will start making time for it.