Because Ranmaru is a yakuza. their marriage was forced in both parties(It's a political/economic marriage). if they divorce, problems for both families will follow, probably even economic problems. since the yakuza group learned about the rumors about Al. most likely they can send some assassin to kill Al. good that Kaoru loves Ranmaru as a brother and friend. still even her warned him about getting together with a man. it's also a problem Al's gender and being a foreign (yakuzas are homophobic and extreme nationalists)
I didn't like the ending but it's realistic.
I mean both Ranmaru and Kaoru are yakuzas and had their roles determined by their fathers since they were kids, or even before born. Ran father is the leader and Ranmaru is a heir to the group, I didn't read about the position about Kaorus father, but most likely he was on tops of the same group or a leader of friend related group, aka an ally group
From what I gather both family's are equal and good friends. The only reason they forced their children to marry is because both were in trouble of shutting down because of money.
Something along that anyway.
In all it doesn't matter. Ran will never sleep with Kaoru. There will never be heirs.
This marriage was doomed from the start because neither wanted to be together sexually. Now Ran's found himself love and he's still meeting him.
Kaoru is not the kind of women to shut up. She loves Ran and wants him to be happy. But they won't be able to sleep together because she won't want to either!
She won't ever have the romance she wants or have children if she remains married to Ran. In less then three years, tops, the Fathers are gonna get irritated that they have no heir. Sure they might threaten Ran, but even if they do he still won't give up his love, and if something happens it won't matter he still will not touch Kaoru. The Fathers are screwed if they don't divorce so Kaoru can marry for love and have a child. That's it!
the story didn't mention the rank of Kaoru's father. can't tell if they were equals or not. and he died. the main reason Kaoru had to marry Ren was because her father died. in japanese society they must follow or they could be banned. what do they feel, DOESN'T matter. in japan they have to do what is told by family. specially when they are yakuzas, politicians, and families of high status.
Al can be killed at any time.
Ran and kaoru can sleep together without loving each other. they don't need love nor idealized feelings for that. a couple of drinks and viola, they can have kids anytime unless one of them were sterile. Ren did said that kaorus was hot when he was a kid.
Al is the only guy he likes. to me he seems like a bisexual guy.
they have to deal with the marriage. kaorus can look for a lover too and have her romance. as the two of them agree to it and keep it a secret from family and acquaintances.
Al also is a captain which send him into looong working weeks. They won't see each other for long periods of time.
plus they met after Ren was married and Al understand that very well and in the position Ren is.
ending is messed up but realistic. is not idealized.
seriously even if they divorce. why Kaoru has to marry and have kids?!?! she can have a great romance without marry. and a kid will only ruin a great romance and steal her personal freedom. Kaoru's characters seems to be the type that enjoy life and parties, a kid will ruin that. no kids please, no more pro natalist overrated archaic propaganda lol lol
So our uke is completely in love with our handsome foreign Seme and the other way around.
Ranmaru and Kaoru are not in love, not sexually attracted to each other, and will more then likely never have sex. Thus never have children. WHY ARE THEY NOT JUST FREAKING DIVORCED?!
The only way an heir is going to be born is if Kaoru get's pregnant by a lover, but knowing how she is she'll want to marry him because she loves him.
JUST GET A DIVORCE! This marriage was doomed from the damn start!