You made me hungry for M&M’s. 

Well said!
But I still think that you have pinned the wrong inner culprit in these relationships.
I agree entirely that many relationships are ruined because there is nothing new to discover about a partner, but I don’t think that the solution is to hold yourself back from knowing that person; that would be like writing poetry slowly so that you don’t run out of inspiration. (think about that one for a second, because I do indeed intend a direct analogy.)

I’ve always found that, in any context (be in friendship, family, or romance) I only find more and more to discover about a person as I get to know them better. People are infinitely complex, and even those who I’ve known my whole life are still growing and changind every day. Even were I as close to another person as I am to my own mind, I would continue to discover that person new each day, just as I see new parts of myself coming to light as I walk through life.

I also don’t think that men looking at the tangible is an absolute, in fact I see it as more a social tendency than a part of men’s nature, but I think it’s quite fair of you to say that it does indeed ruin many relationships; since women generally focus themselves in other directions. I also think that it goes the other way: A man may tire of a woman when he feels he has discovered all there is to experience of her body. But by the same token, just as a man tries to uncover a woman’s hidden body, so will she try to uncover the emotions which he keeps hidden, and just as he will tire of her when he feels he has seen what there is to see, so will she when she feels she has found all of his inner secrets. So men tire of sex with their wives, and women tire of conversation with their husbands, and marriages fall apart to adultery; or simply apathy and disinterest taking its many guises.

It doesn’t really matter in what way a person loses interest in their spouse, the problem is still the same; A decision that that person made in their view of the other. Essentially the attitude that they can understand that person, and then there will be nothing more. Once a man has slept with a woman or performed some act with her, or when a woman has spoken intimately with a man or known his softer side, he or she will often let the sense of mystery go out of his or her time with the other person and the “spark” will go out of their relationship.

But the spark doesn’t rely on any specific act, not on some specific discovery. It depends entirely on realizing that EVERY glance, every moment, is a new piece of this person to be discovered.
People are not puzzles. You can’t collect piece by piece until you understand the whole picture, because the picture is not static.
You will tire of any aspect of a person the moment you feel you have “found another piece of the puzzle,” Because that piece changes with every moment and you’ve let yourself ignore that.

Of course, this problem is only exaserbated by the tendency for men (and often women as well) in our society to idolize sex and to feel that once they have that one experience they will be complete.
But they don’t see that it is just another part of God’s beautiful world, and just another way in which to experience life. Sex can be very special, but it is not the purpose of romantic love. It’s only a happy side effect of something even more wonderful.

In my own life, this leads me to an acceptance that sex is not a necessary part of a relationship. Even if I were infertile and could never consummate a marriage, it would not make a difference one way or the other as to how I look at marriage.
The more I know my own partner, the more I find there is to discover about her. Neither of us feel impeded or held back by abstinence, because we can express our passion without some special act. Therefore we are perfectly comfortable waiting for marriage.

Also, and most importantly, I find that as long as I look at her, and at the relationship, not as a puzzle to be solved or a challenge to be conquered, but as a garden that is always growing in new and wonderful ways, I see that the more I discover, the more there is to discover. So we know one another more the longer we’re together, and also find more mystery. This creates a spark between us that won’t get old as we grow closer, but only grow stronger and brighter with years.