Well, I would say that it certainly does happen, and probably very frequently. The one caveat I’d add to probably get closer to your intended question is that the sexless love be would exist throughout the relationship. I say this because I wager many, and possibly the majority, of elderly couples live in a unquestionably loving relationship which lacks sex. The lack of sex may be bacause one of both parties no longer are hormonally/physically able to have sex, or due to simple aged lack of lust between to old frail bodies. Its certain that the vast bulk of couples began as two highly sexual beings, regardless of if it began prior to the sexual revolution, but the simply out grew it. I doubt this exists as part of your query.
A second situation is in the case of a couple maintaining a relationship for the sake of their children. A couple could begin hot and heavy, deep in love and lust, and have children along with the white picket fence, but for one reason or another grow apart later. This couple most often remains loving, but not passionately so. Outside circumstances like children certainly put couples in relationships lacking sex.
Finally, there are also cases like evilbeth described in which one or both parties are emotionally unable to enjoy sex in a healthy way. In such cases love and sex are necessarily divided.
So, I’m going to assume that you are looking for cases where a couple is not interested in sex, in spite of the fact that they are deeply in love when the relationship is in its simpler early stages. Perhaps where the two parties are simply not physically attracted to one another, but connect on the most fundamental of emotional levels. This is indeed rare, nearly unheard of.
The reason for this is absurdly simple in my eyes. Humans are animals, driven by all the lovely survival instincts. We absolutely must eat, drink, have shelter, and reproduce. Overlooking various statistically rare mental and emotional flaws, these factors are never usurped. No where in that does love come into play. Sex, however, is quite critical to maintain the species, and every living thing’s main purpose is to propogate itself. Food, drink and shelter are simply steps that must be taken in order to facilitate the propogation of the species. Sex, ergo, is our number one fundamental drive.
Love is a social creation which intelligent (and perhaps present in other lower mammals) humans have developed as a need. We don’t really need love, but eons of human evolution has conditioned us to see it as one. You might argue that if a man were to grow in a isolated surrounding, in which he were not presented with love as a critical element to life, he would never believe he was missing anything without love. As long as he had a female to copulate with, he may never know he’s missing out on anything. Why, as a society, have humans grown to need love? Well, I’d guess that it simply provides a more opportune situation to get the other 3 needs. Its simpler for a man and woman to share the feeding, and shelter responsibilities than to exist seperately until its time to meet for sex. Men and women have different assets, and a team effort raises the likelyhood of surviving to reproduce effectively. As such, love is a manufactured need. In my opinion love exists to facilitate effective sex.
If you agree with this interpretation, then its clear that sex is a more fundamental need than is love. In life biological needs almost always override psychological ones. I think this circumstance makes love a moot point without sex.
All this generalizing aside, there are certainly person to person variations which aren’t going to fit the strict scientific mold. This would explain why there are anorexics who overcome the need to eat, homosexuals who overcome the need to reproduce (no moral comparisons intended), and suicidal folks who simply brush off the survival instinct altogether. These human tendancies are going to account for the cases of sexless love, cases where our intellect overcomes our base needs. But I think when you look at the big picture you can see why casual, loveless sex is so much more common than sexless love.
Thank you all for responding to my nocturnal wonderings. It’s nice to finally have a place to discuss them (though I’ll try not to ask too many of them; wouldn’t want to overdo it).