Have you tried any of those dating/meeting sites, like DatingGold and etc.? Failing that, i can only really think of some AdSense alternative, but as you said, they will be acting as middle-men (although AdBrite admittedly acts more as a broker than anything else).
Maybe you could just smack up a copy of phpAdsNew and sell advertising space off to interested parties, at least until you come up with a specific product to promote.
