I've only ever seen this addressed in a book ONCE, and it was just a few lines. Very frustrating, isn't it, for those of us who live with this? However, I did think the brief commentary I read was on the mark. It's been a while, so I'm paraphrasing here, but I believe the author mentioned that depression, low self-esteem, and stress in general could shut down a man's system. 

For my husband, I believe this really is at the heart of the problem, not any physical issues. My husband is very focused on making sure I get to stay home with the kids, and with seven kids at home...well, that's a lot of pressure, ya know? He has a very introverted, laid-back temperment, so stress doesn't show so much on the outside, meaning, he doesn't get super grumpy or mean or do anything much different from his normal routine, he just starts pulling away and shutting down. 

He also has some traumatic things in his past (not related to sex) that I don't believe he has dealt with, and I think that has leaked over into this area. When we don't allow God to help us heal, we tend to build walls around those areas in our lives, and it's only logical to expect that it would eventually affect us in other ways as well, yes? 

As humans, we are creatures of habit, and in addition, we tend to take the path of least resisitance. I think part of our problem is that, after so many years, it becomes "easy" to continue in our habit of infrequent sex...to not make it a priority to work through it, because it is uncomfortable, and because for some people, it's easier to avoid it than it is to risk feeling rejected or like a failure. I have been guilty of this as well--sex that ends in tears (mine) is really a downer lol.

A few things I have learned:

Don't Take it Personally. 
Evaluate your behavior, ask God to convict you of anything that is your responsibility to change, and then beyond that, try not to let your feelings be hurt. I know how hard that is to do, but it just zaps your energy, and it's not constructive.

Remember: This, Too, Shall Pass. 
One way or another, things will get easier. For me, my desire has become much less frequent than it used to be. I no longer cry myself to sleep because my husband "doesn't love me enough to want to be with me," or whatever. I know he DOES love me, and he DOES think i'm a hot tamale; he just has Stuff, and we as a couple have Stuff, that gets in the way.

Don't Lose Hope.
This is probably a season, although it may seem like the eternal winter in Narnia, with nary a sign of Christmas. Be confident that God intends for your marriage to include a satisfying bedroom life, and that He will continue to work toward accomplishing that end. Pray for your husband, your marriage, yourself. Pray that God will keep your heart soft toward your husband no matter what his behavior, or your perception of his feelings about you. Pray that God would enable you to meet your dh's need whenever he approaches you, with a cheerful and sincere heart. (That last bit can be a bit tricky, as I discovered that after I finally felt like I had put away my desire in order to weather the storm, husband would approach me at the most unexpected times, and I'd be filled with anger and resentment. Here I'd been trying so hard to stay turned "off," and then he waltzes in and wants to get frisky! It was a good lesson for me--God wants me to be Available, not turned "off." Hopefully, you aren't making the same mistakes I did.  )

It's Not the End of the World, and, Rome Wasn't Built In A Day. 
Yes, marriage was meant to include sex. Yes, if there is healing to do, regardless of whether the issue is physical or mental or spiritual or whatever, it will probably take a while. Don't let this issue consume you, don't let it take over your marriage, don't get stuck in it. Work at honoring your husband and obeying God, and look for long-term blessings, not a short-term fix. As we give grace and mercy to our husbands, we become more aware of how undeserving we are of God's grace and mercy toward us. And, at least for me, of my husband's grace and mercy toward me in other areas. 

He Gives Beauty for Ashes, Joy for Pain.
Remember, God intends for this trial to shape you into being just a little bit more like Jesus. There is a purpose in all of this. Allow Him to smooth out the rough spots in you, and thank Him for loving you enough to not leave you the way you are. 

One last comment:
Counseling can be very helpful, if your husband wants to go, and that's a big IF. In my experience, most men would rather have a root canal. Don't set your heart on counseling as being the magical cure-all for any problem, ever. If he wants to go, great. If not, do everything you know to do, and trust God, the ultimate Counselor, to be sufficient for all of the needs of your marriage. 

Be encouraged--it ain't over 'til it's over, and you never know what God's got planned in the meantime! 

{{{hugs}}}