I'm in the same boat as you. As a matter of fact, I'm pretty much done trying to touch her. A man can only do so much before he says "forget it"
Double zombie thread! 2012, 2013 and now 2015. Impressive!
In any case, this topic is repeated numerous times here on TAM, and in between the "she's cheating on you" and the "she's just not into you" replies, there's one solid piece of advice that fits:
Quit making a big deal out of it with your spouse. Whatever the reason for the lack of sex to creep into the marriage, putting all the pressure on ones spouse makes it worse.
Whether one likes to hear it or not, 9 x out of 10, it's a "you" issue (even if you don't think it is). People generally don't want to have sex with somebody they're not attracted to, and attraction can be fleeting, and it can come and go. What somebody found attractive about you 5 years ago, could be boring to them now, or they could simply take it for granted.
As well, being with the same person for x-number of years (and only them) and/or always having the same sex, same positions, initiating the same way, on the same days, etc. leaves little room for excitement.
My wife and I try to mix it up as much as possible (and I still have some work to do with this). When we were first together, she had never had sex with someone the way she had sex with me - in other words, she was used to not having much attention paid to her, and otherwise relatively quick sex (especially with her ex, I learned). For the previous 4 years of her life before me, she was lucky if she had even one orgasm during sex, and it was always more or less the same every time.
With me, right off the bat, I paid attention to her, did new things, took my time, and ensured she enjoyed herself, too.
Then 4 or so years into our relationship, I found myself in the same boat as the original OP, as well as you, and wondered why. It took some time and some communication, but generally speaking - same problem, it's always the "same". And as crazy as I thought she was at that point (I mean come on, I was making sure she had multiple O's, I paid lots of attention to her, I was doing the things she liked, etc.) - it WAS always the same.
So we figured that out, and now we mix it up a little bit, but I do still have trouble just banging away sometimes (which is what she actually wants from time to time. Go figure.) I've learned to read her, and I'm usually successful at determining if she wants a longer session or something shorter, if she's good with one O, or in the mood for several, etc etc etc. But my issue was that I wasn't doing that for years, and I just assumed she always wanted like 4 or 5 O's and this or that.
So no matter how good the sex is, theoretically, people don't always want it that way. My ego got in the way, as I thought I was freakin' awesome in bed and she'd want it that way every single time until we're too old to have sex any more.
And believe me, this scenario is worse if your partner isn't even satisfied with what you're doing (been there, done that - ex wife).
So what I've learned is that people want variety. We all do. As most of us take our marriage vows seriously and don't go and get variety elsewhere, it's up to both parties to "mix it up" a little, imo. If you're lucky enough to be physically sexually compatible with your spouse, you can stretch it out much longer than if you're not, but you usually end up at the same regardless, if you don't incorporate some variety into your sex life.
And the same goes for your marriage, too, outside of the sex. It's perilously easy to get into the mindset of "you again?" If it's the same old, same old, day in and day out, it's not difficult to lose interest in one's partner. I am currently viewing my wife that way, as she's on quite the schedule these days. She and I haven't had any time to ourselves in a few months, and her schedule is the same every day. But it'll change, and we'll get out of this rut. Ironically enough, it's made me not be as interested in her sexually lately. Not her fault, it is what it is, but we'll get out of it.
Also, to make a long thread even longer, the worst thing one can possibly do when the sex has slowed to a trickle is to complain about it and put all the pressure on the one spouse. I learned this the hard way, too. You want to be making yourself more attractive to him/her, not less. And by whining, getting angry or frustrated, or otherwise putting it all on them, that's exactly what you're doing.
It's all the little things outside of the bedroom that will get you some action IN the bedroom. When you meet someone and start dating, you show the best side of yourself and make all kinds of efforts to be attractive. People often lose sight of this once a relationship gets serious, and especially after marriage. But marriage still requires one to maintain the attractiveness - from both sides. So in this case, and most of these cases, the one person has lost site of this and no longer feels like they have to put forth quite the same effort as they did in the courtship phase.
I did this with sex. I was so good at it with my wife, that I thought all I had to do was maintain that level for our entire marriage, and I was good to go. She'd want it all the time, and how could she not? With her, it became an ego thing for me - something I knew I was good at - so I don't have to change it up, ever. (and trust me, my wife is the only woman I've been able to fully satisfy in bed like this, so it's not like I think I'm a super stud or anything.) And here's the really ironic thing about our "issue" - my wife did not WANT all the attention paid to her after a while. Although she physically enjoyed it (and still does), from time to time she wants me to just take her, bang away, and enjoy MYself, rather than spend so much time worrying about HER pleasure. And I actually get that mindset. From time to time, I wouldn't mind my wife just jumping on me, riding away and having a good, hard orgasm without all the other stuff.
The trick is, obviously, to not do that every single time. Just as she didn't appreciate the old "jackhammer" every. single. time. from her ex, she also doesn't want the complete opposite every. single. time. from me. Go figure, right? But you know what? I wouldn't want that, either. My wife is very good at pleasing me, but she doesn't always do the same thing. No matter how good she is at oral, I probably wouldn't want it every single time as foreplay, either. You'd get to a point where sex is basically scripted, and both people know what's coming next. And that's more or less how she felt. She knew what to expect. No matter how good it actually felt, it gets into your head after a while. "Oh, he's going to do this now, followed by that, and then the other thing." Wash, rinse, repeat.
In the last year or so, some of the best sex she's enjoyed has been quickies, or me skipping a step or two, or just getting right to the point. Psychologically, this seems to have done wonders for her, and I suspect it actually makes her feel more wanted and desirable by me.