One major complaint from husbands about their wife that I step over again and again is that she seems not as sexually available as he would like her to be.
Men then quickly assume that their wife "just doesn't need it". Bad luck. With it they get their believes from teenage-years reconfirmed that "Women are just not as sexual as men."
Well, here's my take on it: It is a reality that women are conditioned to hide their sexual urges, especially when they are young. And that for a good reason: There is a - very real - social punishment awaiting those who are labeled a "slut". Those women, although they appeal very much to a man's short-term sexual strategy, do have a - very real - critically reduced chance to find a life-partner of high genetic quality to mate with. Her instincts are fully aware of that fact.
(By the way, a modern woman's independence and newfound sexual freedom does not make this instinct go away!)
Therefore, being a "hard-to-get" is a powerful and important strategy for women, not only to test the man upon his sincerity, but also to put her own qualities in the right light for potential mates. It helps her to move up in ranks in the genetic pool.
But once in a secure haven (usually marriage), a woman should be able to unleash the inner sexual demons. For that to happen, however, two crucially important things have to be established in your marriage. If your wife still continues to have reasons to avoid being intimate, there is a good chance you fail in either one or both of this two areas:
1) You've got to be sexually attractive. Yes, this is about your physical qualities, but not only (and most often not even mainly). You can flick a woman's sexual attraction switches even quicker through certain character traits. Many men misunderstand that. It is essential to learn what it is that your wife feels sexually drawn to - and know that there are lots of things. If she's alive, she is sexual. We are genetically programed that way and it is natural to all of us, men and women. Be, however, aware of the fact that women often marry men they do not feel sexually attracted to at all in first place. It is not necessarily a primary condition for her to get involved with you.
2) She must be able to have the unshakable trust in you that opening up to you sexually does not have bad (genetic) consequences for her. This also includes, instinctively, to keep her options for serious high-quality mating alive should you ever for any reason decide to run out of her. One of the top qualities in this resort is a strong personality that is unaffected by others (even by her). Wives test their husbands every day without the guys ever realizing it (therefore most of the times failing it).
If you can get those two aspects right, your woman will go anywhere with you, also sexually. If not, no pleading, begging or even threatening will ever help you.
By the way: "Opening" a woman "up", sexually, is a skill that has really very little to do with how sophisticated you are in licking her clit, finding her g-spot or the size of your penis. If that's what you are working on, you're in the wrong corner.
Find out how to become more sexually attractive yourself. Then get her to trust that opening up to you is safe thing for her to do (genetically). When you find that position with her you will quickly realize that you do not have to "sell" her anything anymore. She suddenly wants to "buy".