Something like Sanctuary, Sun-Ken Rock, Eden:It's an Endless World and the like. I really like the idea of an attraction so strong that it transcends something like traditional limits.

Any suggestions?

EDIT: Sorry, "TWO" instead of "TO".

Although it is not categorized as romance, Black Lagoon has two main characters that are yin-yang to each other. Revy is the downright ruthless killer while rock is the peaceful negotiator. As you read through the manga both of them develop a deep relationship though it is subtle. For example, the cigarette kiss or the gun and bullet analogy is tantamount to what you call romance in my opinion.

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Thanks for the response! I'm actually a hard-core Black Lagoon fan. In fact, I've asked around for something like BL a few weeks ago, too (mostly, the closest match would be Jormugand). Yeah, BL is also quite good, and Revy and Rock's romance is amazingly and beautifully played out (though I'm depressed since the manga is on hiatus right now, and I can only subsist by reading the light novel).

It seems we have similar tastes... any other suggestions?

Well it is an anime:
Jin-Roh: The Wolf Brigade