All good answers above, so I will not elaborate but simply tell you that Keeping Sex Lively in Marriages is a common problem and concern ... most couples hit that point (to greater or lesser degrees) at some point in the marriage. Just like eating a great steak, if you did it everyday it would lose some of its appeal after while; that's just basic human nature.
Therefore, smart couples realize this (sooner or later) and they do various things to "vary the menu" so to speak (and no pun intended there). That is what all the previous posts are suggesting... METHODS and TECHNIQUES to bring variation and fun back to marital sex, to overcome "same ole, same ole" effect (that otherwise tends to be inevitable).
SUGGESTION: There are many books on this top, some by sex therapists and psychologists and psychiatrists and some good books by others on the topic. Go to a book store and browse, pull some off the shelf and sit and read them and find the book(s) that work well for you. Note the big bookstores (Barnes, Nobles; Boarders; Books A Million and more) make it comfortable for you to sit and read their books before (or without) buying them.
Also, do search on the internet as there is a lot written on it on the internet too. If you want scholarly, detailed and more technical explanations, you should do research of SCHOLARLY ARTICLES (contact your university or library and ask them to show you how to do that, if you don't know how ... those sites are not so readily available on the "web" but you can get to them).
Don't be inhibited; you and your husband should make this a project: How to Improve our Sex Lives. If he is not interested in that, then you do have a problem and another kind of book and therapy would be called for. Mostly likely you can get him "into it" without too much difficulty, if you take the lead.
Good luck,
ROB