Hey guys, so I'm looking for some suggestions on a manga with a realistic progression of the relationship, or as realistic as it can get.
I've read way too many yaoi lately where any kind of reasonable development is tossed out the window in favor of sex. Personally, I find the sex more fun AFTER a decent build up of the couple.
Also it would be a bonus if the manga depicts a couple already in a relationship. I love reading about relationship issues.
P.s. I don't mind shounen-ai either.
Thanks in advance!

Sorenari ni Shinken nandesu. is a classic

i have a liking for slow romances and realistic gay relationships too, so here are some my favorits:

Arashi no Ato
Complex
Dear Green: Hitomi no Ounowa + sequel
Hana wa Saku ka
Haru o Daite Ita
Ikumen After
Kage Aru Tokoro ni
Kinou Nani Tabeta?
Kyuuso wa Cheese no Yume o Miru + sequel
New York New York
Sakende Yaruze!

Blue Sky Complex is definitely worth checking out! It has great development and the characters are quite interesting. The art is nice and Slow (ICHIKAWA Kei) is another good read from this mangaka. Number Call has a realistic take on same-sex relationships as well and the art is really pretty. I loved this one so hopefully you will too! (:
Here are some others that may catch your interest.
Ikusen no Yoru this one is great for relationship issues.
Endou-kun no Kansatsu Nikki
Oh and i second Haru o Daite Ita and Kyuuso wa Cheese no Yume o Miru if you love complex relationships.

lιvιɴɢ ιɴ α yαoι woɴderlαɴd
-QaF-

Nemureru Mori no Binan is as realistic as it can get. I didn't enjoy it at all because the characters are practically impossible to like, but...

Most of Ono Natsume's BL works and some of Yamashita Tomoko's titles, especially Illumination and Kuimonodokoro Akira.

Nishida Higashi's supposed to be good about this, too, but I can't be certain because I haven't read anything by her yet.

You might also want to try some of Ima Ichiko's BL titles.

Zankoku na Kami ga Shihaisuru... Maybe. It's difficult to say because the circumstances of the plot aren't normal at all.

And, as mentioned before, Kinou Nani Tabeta?, Kyuuso wa Cheese no Yume o Miru + Sojou no Koi wa Nido Haneru, and Hana wa Saku ka definitely fit the bill.

Komatta Toki ni wa Hoshi ni Kike! (https://www.mangaupdates.com/series.html?id=515) is one of my favorites of all time. (WARNING: This is like the ultimate shounen ai but it's totally worth reading). You can really understand how the two leads are soul mates and how their relationship evolves. If you don't mind 26 volumes.....

Let Dai (https://www.mangaupdates.com/series.html?id=6707) has a pretty realistic series of events. The art is okay but the story line is mindblowingly good; There's major character development and the romance between the two leads is tragically beautiful. If you don't mind 15 volumes, you should definitely read this manga. 😀