
this is a continuation but shunna is looking back on the time before he and kanchan starting dating

No it's a direct continuation after the photo was taken. Life isn't 'fixed' and happy and perfect just cos you admit your feelings and start dating. Granted normal honeymoon periods last longer, but these 2 have a few issues between them, domestic abuse history and straight to gay. Which is why the story seems darker.

wait but when he talks about how he only thought of kanchan as an "onahole" but then found himself "seriously falling in love" i thought he was thinking back to before they starting dating
only the end of the chapter is present time?

I got that too. Actually, it seems he goes back and forward.

The translation is confusing but what I get is this 1st chapter is a brief summary of the prequel told from Shuuna point of view. A bit of Shuuna's high school days (seems like he's popular with girls), the start of Shuuna & Hayashida's casual relationship (sex friend), Shuuna started to fall for Hayashida, then back to the present. You can see in the flashback there are only that one photo of Hayashida smiling widely from his high school days and when the story transition back to the present there's additional photo of Hayashida and Shuuna smiling. I'm guessing, now that Shuuna and Hayashida become lovers the story will focus on trials and tribulations of an established relationship. And it seems Shuuna have some insecurities because Hayashida didn't speak much about his past. Plus the unexpected transfer to Osaka that surely will put some strain in the fairly new relationship.
I don't understand if this is a flash back or if this is the continuation.. because at first the photo of them laughing there isn't... and the seme (I don't remember his name) seems not knowing that the uke hit his ex boyfriend... mmh...