"Moving on to a deeper kind of love." ROFL. That's a riot. SOMETHING is deeper, all right, but it ain't the love. *giggle*
I went through that sex once-a-month thing.
I even made some of the excuses that your partner made.
It's a load of hooey.
The way you describe it, he has no interest in seeing to it that you have a good time. He's basically enforcing his (false) viewpoint about sex on you without considering your feelings at all. So how is this man someone you consider to be "sensitive"? He's sensitive to his own "needs", sure. He's not sensitive to yours at all. Even though he's tried to slow things down at your request, he's only doing it to compromise, without really wanting to rock your world. And, what's worse, he's trying to convince you that your feelings about sex are wrong. You think that sex should be an expression of love. He's turned it into an expression of animal release, at best.
Tell me, is it true that "it's just the natural state of things to have an intense sexual interest in one's partner for the first year or so, and then gradually lose interest over time"? Do you *really* believe that?
I promise you, it's only true if you believe it.
And, for him, I'll bet that he *wants* to believe it, because he doesn't really want to look at what's gone wrong in the relationship.
Is this man your best friend? Can you tell him anything and he's OK with it? Can he, does he tell you everything? You know, like you would with your best friend? Even the embarassing stuff? The bad stuff? The stupid stuff?
When you reach that point with your lover, where you both have a completely honest, open, free-communication relationship, you'll never again believe that what you're experiencing with your current lover is "just the natural state of things". It's not. When your best friend and your lover turn out to be the same person, then your interest in sex and the quality of it increases. It doesn't lessen, it grows.
Unfortunately, I don't know what you can do to change it. You both have to reach a point of wanting it to change. As a shared commodity, sex just doesn't get very far without the willing and active participation of both parties. I've not yet met many folks who are willing (or even realize they can) fight for it, to make that jump to "best friend AND lover".
It doesn't just happen. It has to be created. And, from what you've stated so far, he doesn't seem to be willing to create it, and, instead, is willing to make up excuses for why it has to get worse.
(In fact, neither does uninteresting sex "just happen". It is also created, regardless of how much the parties involved want to pretend otherwise. If sex is not the amazing mind-blowing make-you-believe-in-God kind of act you suspect it can be, then know that one or both parties are actively preventing it from being that, whether they realize it or not.)
You might ask yourself "Why?" To which I must ask, "Does it matter?" Is there some reason that he can give you which you would accept to make this situation OK to endure? Any one at all?
What if you were in his position, unable to express love for whatever reason, believing in all the reasons why the sex has to deteriorate, basically treating your partner's wishes as irrelevant. Would you want him to stay with you while you treated him that way? If you really loved him, could you possibly want that?
It isn't just about the sex, by the way. It's about not having your best friend as your lover. Sex is like a yardstick that you can use to gauge the state of your relationship. If the quality of the sex is increasing over time, then your relationship is growing. If the sex is getting worse (or has reached what you hope is rock bottom, 'cause any worse would be degrading...), then your relationship is falling apart (or fell apart long ago and you just chose not to notice...). It's easy to focus on the sex, which is just the expression and measurement, and buy into the whole "deeper non-sexual" stuff that he's offered as the reason, but that argument is just an excuse for him keeping his distance. From you.
You deserve to be happy, unless you decide otherwise. You deserve to be able to share with someone who wants to share back, unless you decide otherwise. So... decide. Is this man worth sticking it out for?
He could very well be "the one".
But if you take a deep breath and look at your situation as an outsider -- not caught up in that smile that still has you dazzled -- and you don't foresee anything but being 50 or 60 and feeling like you don't have what you want out of this relationship, then moving on is really the only viable alternative.
So many choices...