Kaichou wa Maid-sama!
What I mean by "legitimate progression in the relationship":
When they take one step forward, they don't take two steps back
(ex. when they finally kiss, they/the girl doesn't regress back in to self-conscious "he doesn't love me" fits)
Other than that requirement, I'll say I'm open to any genres except Shoujo-ai/Yuri/Hentai (though I highly doubt there's a Hentai with enough plot/"relationship" to satisfy this condition anyways)
This means I'm fine with yaoi

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From the manga: Good Luck, Demon King!