Perhaps An Insight Into the Guy's Problem...
All of the stories here are all too familiar. What's frustrating is that they all reflect a fundamental change in the relationship early on in a marriage...which fortunately, is something I might be able to help out with.
Many guys get married before they really understand what it is to PARTNER with someone. I'm not talking about the same-sex "partner", but the real joining and combining of lives that is absolutely necessary in a marriage. This kind of change takes a lot of courage to enter into - if you're not ready or aware of it. What typically happens is that within the first few years, the couple who has previously been on individual paths and decided to get married, now finds that they are having to be on the same path - and that means compromise, a lot of it.
For many people, that kind of change becomes associated with a loss of the self. I had wanted to be a police officer, but my wife convinced me that a dangerous career was going to cause us problems together, so I gave that up. I no longer regret that, but it took time. The same went for her having to give up living in a big city to live a more suburban life to follow my job. In all of that, you can start losing your personal identity.
Sex is an expression of identity, of the self, and it's best when you aren't thinking about change, aren't worried about bills, responsibilities, or whether or not the sheets you just purchased represent an emasculation. Married sex has baggage just like all sex does - but it's different baggage. For men, the loss of independence in life can be a bedroom killer. If they don't feel in control of their destiny any more, and they blame that on you, then why would they feel sexual? They don't .
I'm not saying this is good, or that anyone should just lay down and never change. What I'm saying is that it's everyone's responsibility to recognize that change and compromise MAKE and DEFINE marriage. You don't stay married for 13 years without fights and disruptions, and you don't weather those times without compromises both big and small. The problem, then, is to remember that your sexuality, virility, and "manliness" is not an expression of your individuality or independence as much as it is two people in love sharing that moment, or just scratching an itch.
For the guy, however, if effort is no longer being put into sex, if his partner isn't "showing off" as much, or both have become less-attractive to each other as a result of "relaxing" - you're no longer dating, so you don't have to try as hard, yeah? No. But that's a common pitfall.
What can really help is to help your husband find and clarify the parts of his life that are still absolutely his - how he dresses, his routines, how he works out, what his job is, whatever... - and then show him how absolutely essential he is to the functioning of your family (with or without kids), then he can start building a solid sense of self and individuality within the context of the marriage partnership.
We're babies for life, most of us, and we need to be told a lot of things to make us feel good about ourselves - not that we're pretty, but that we're strong, capable, your hero, and all of that. Once we lose the feeling of being the hero in the relationship, it's hard to get that back, and it's doubly hard to recover from it.
Hope this helps.