Justin Bieber's exes aren't don't seem to be phased by his recent possible MARRIAGE to fiancee Hailey Baldwin ... Sofia Richie and Chantel Jeffries are just goin' about their business!

Of course neither had a real chance with the "Girlfriend" singer, right? A twenty-sometime pop star -- like the most famous pop star in probably all of the world right now -- is allowed to have some decidedly un-serious flights, right?! And Sofia and Chantel were just that.

If you remember, Justin and Sofia -- then a very young 17 -- started dating in August 2016 and lasted for only a month. Fans and even Selena Gomez slammed the singer for robbing the cradle and just like that, it was over.

Biebs and Jeffries have a bit longer history together. They started hanging out in 2014 -- she was even there when he was arrested for DUI in Miami in January that year, then reunited for a bit in 2016.

Of Justin's other exes, we're pretty sure at least one more of them gives zero Fs about the singer's reported marriage to Hailey ... the significantly older mother-of-three, Kourtney Kardashian!