Best example of what I'm looking for:
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 😉

From the manga: Good Luck, Demon King!

Yandere Kanojo Don't let the "gag" label scare you off. It has more plot, especially in the relationship part than most manga focusing exclusively on relationships.

[img]http://www.mediafire.com/convkey/a8ee/v1c0uzwpt6cdtcbfg.jpg[/img]

Quote from bajuwa

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 😉

Actually, there's plenty of plot pron... good plot porn, too. And even moreso in the Borderline Hentai category.

Although borderline tends to have MASSIVE stepbacks when it's got a nicely complex and very realistic plot.
Example: USOTSUKI PARADOX (stupendously realistic, serious, and enticing plot, possibly the best depiction of mature love and yearning I've ever seen, and some of the tender-est love scenes ever... but she dumps him. and dumps him. rips out his heart and sends it through a juicer. and then dumps him again.)

I'd definitely recommend Horimiya 😀
It's such a light hearted romance and the characters are so lovable!

Bonnouji

in need of romance?
uncommon, sad, sweet

Quote from keshant

I'd definitely recommend Horimiya 😀
It's such a light hearted romance and the characters are so lovable!

But there's no progress.. yet so they can't even step back xD

// not entirely true actually

There's some progress but they're still only friends

[img]http://www.mediafire.com/convkey/a8ee/v1c0uzwpt6cdtcbfg.jpg[/img]

Well, the most recent one I'm reading is Akagami no Shirayukihime. Went back to rereading it now that a few chapters stocked up. It's not really a difficult requirement to fulfill... so tbh, it doesn't really stay on my mind.

Hum... a quick scan through my list I'd suggest Kanojo wa Uso wo Aishisugiteru (very annoying manga... but nevertheless good) and Love So Life.

[img]http://www.nodiatis.com/pub/19.jpg[/img]

Natsu no Zenjitsu
Bonnouji
Horimiya
Love Roma
Kare Kano

KareKano?! Whaaa... THE archetypical 100-chapter shoujo as an example of relationship progress? 😕

Mars. The romance development is more mental than physical, they slowly get to know more and more about each other and their lives. Only once they have seen each other at their best and worst, does their relationship get properly physical, and that doesn't take over the entire story.

Sora Log. I can't remember much about this one, but I remember I liked it.

Strobe Edge. The romance here is very slow, only nearing the end does the couple get together after becoming good friends first, and then becoming girlfriend and boyfriend.

Dear green: Hitomi no Ounowa
Dear Green this is a prequel and a sequel, the romance is slow, realistic and natural. It was the first yaoi I read, before then I was pretty grossed out at the thought of it.

I would suggest looking up manga with the category of slow romance, friends become lovers, character development, realistic relationship or any other categories you see popping up allot in your more favourite manga. You can made the search term smaller by putting what genres you want in as well.
Good luck.

Also, you are welcome to look on my complete and reading list, my favourites tend to be slow romances with something more than just stupid female leads. That said, our ideas on what is intelligence may differ, therefore, you may find my taste in manga, not overly appealing.

We work in the dark, we do what we can, we give what we have, out doubt is our passion and our passion is our task, the rest is the madness of art. Henry james

Quote from Lorska

Bonnouji

great recommendation! awesome series and meets the requirements perfectly!

From the manga: Good Luck, Demon King!

Quote from RattixEmpire

I would suggest looking up manga with the category of slow romance, friends become lovers, character development, realistic relationship or any other categories you see popping up allot in your more favourite manga. You can made the search term smaller by putting what genres you want in as well.

Both in response to this, and also to anyone else who looking to recommend:
I'm not looking for a "slow romance". If it takes them 100 chapters to get their first kiss, that means there was little to no progress, and that's just as bad as backwards-progress.

From the manga: Good Luck, Demon King!

Suzuka

Kiss by MATSUMOTO Tomo
As the story starts with the main couple getting together, this early work won't leave you waiting for progress. Even though there is a physical aspect to their relationship (and quite an affectionate one at that), it focuses on their growth as a couple. The mangaka captures the difficulties of being apart in terms of age in a tender, yet convincing way, and as they come to understand and to define their own roles in this relationship, they build this elusive thing called "love".

In addition to the manga, there is a list for all the music mentioned or performed:
KISS (Matsumoto Tomo) - Music from the manga

[color=#8A795D]"Stories are what death thinks he puts an end to.
He can't understand that they end in him, but they don't end with him."

• Ursula K. Le Guin, Gifts[/color]

To be savoured:

• Blood Alone by TAKANO Masayuki
• Otoyomegatari by MORI Kaoru
• Gangsta. by Kohske
• Seishun Kouryakuhon by AKIZUKI Sorata

Maybe Monkey High

My current top 3:
Fullmetal Alchemist
Pandora Hearts
3-gatsu no Lion