My impression, this is not about discovering they are gay.
It's a very beautiful story about meeting your out of time wife. A full slow burn romance from the very beginning when characters are still unaware of their fated love, revealing the mystery of their connection.
I also think there is something "paranormal", like appearance of Yoshiro who could be their daughter from the future, a fortune teller like some higher guidance who gives direction to Adachi how to fulfill the fate.
It's up to her to initiate, she is more aware than Shimamura who needs complete awakening to remember their "love written in stars".
It looks like awakening of sexuality, but I would not agree. I see the deeper meaning in their relationship.
Everything in this manga is thought-out and very deep, even if it looks like nuisance sometimes.
That's how I get it. I could say more, but this is too much info already.
I did not make a comment for a long time, just reading around, but now I was moved...
If someone already expressed same thoughts about this before me, I really did not read all comments here but I would be glad if there is more similar opinions to mine.