I'm looking for romance manga where one or both the main leads are already in a relationship at the beginning, but they begin to fall for each other instead. E.g. Sprout.

Thanks in advance~ :3

Koibana! Koiseyo Hanabi
The One << u can see it around chapter 15 -sort of-
The Bride of the Water God << probably not what u r looking for but(there is another girl, oh yeah XD)
Atashi no Okiniiri

** "It's so warm that, Before the snow has a chance to stick, He melts it for me." **

I've read The One and Bride of the Water God, but I'll be sure to check out the other 2 titles, thank you ~~

In Koibana! Koiseyo Hanabi, the male lead is currently dating someone else and has been since the begining of the series (it's only 3-4 volumes so far). To be honest with you though, I can't fully recommend the manga because I despised the mangaka's previous work, Parfait Tic. (It started off very strong but became awful in a big way in the latter half of the series, though many people do like the series.)

There's also Bitter Virgin (a manga I can recommend) where the male main character, due to circumstances, is dating someone else, eventhough he'd rather be with the female lead.

Also, Do-Re-Mi-Fa-Sol-La-Si-Do, a manhwa where (for once) the lead female is dating someone else. (It's on my to-read list as a lot of people love it.)

Maybe try Be My Sweet Darling, another manhwa where the male lead is initially dating someone else? (Though circumstances quickly change that.)

Perhaps also try Hachimitsu to Clover (Honey & Clover)? It's an ensemble story, but there's a number of missed-connection romances in it. (It's somewhat of a realistic story line as not all of the people in question leave the people they're dating for the person who is pursuing them.)

A plotline like that happens in Suzuka (which I haven't read) and School Days (which is a rather infamous manga). I'm pretty sure Nana also features that plotline as well.

Finally, two series that I really love which are about missed-connection, second-chance romances are Atashi wa Bambi, a manga about a girl who get a crush on a classmate, but discovers things aren't as easy as she might like them to be in gaining his affections, and also the manhwa Replay, which features missed-connection love through the years. Both series do feature a main character dating someone else, but the outcomes aren't as clear as one person dumping their boy/girlfriend in favor of the main character. (It's a bit more complicated in both series--probably why I liked them so much.)

brid, thanks for such a detailed reply <3 You've got me very interested in Atashi wa Bambi and Replay -- I will check them out ASAP. Thanks again ^^!