That was quick! Did you wonder why Liam Hemsworth filed for divorce from Miley Cyrus so soon after they announced their separation? Well, it's kinda obvious, actually ...

Liam was PISSED when Miley was photographed making out with Brody Jenner's ex Kaitlynn Carter on a boat in Italy. A source tells Us Weekly Liam was upset that Miley was "being so open about her relationship with Kaitlynn." The source went on: "He thought they could work it out, but all those pictures of her and Kaitlynn ended that. Liam comes from a very conservative family and his family was freaked out by it."

Freaked out? Do they know who Miley Cyrus is? She's been "out" as bisexual for years and she's expressed openly her love of marijuana and experimentation, etc., etc.

It was Aug. 10th at the couple, through a rep, announced their decision to separate. Hours later, pix of Miley and Kaitlynn kissing during their vacay in Italy were published and then on Aug. 21, Liam filed for divorce.

Social media immediately exploded with speculation and accusations that Miley must have cheated on Liam to which Cyrus responded on Twitter, "Once Liam & I reconciled,I meant it, & I was committed. There are NO secrets to uncover here. I’ve learned from every experience in my life. I’m not perfect, I don’t want to be, it’s boring. I’ve grown up in front of you, but the bottom line is, I HAVE GROWN UP. … I can admit to a lot of things but I refuse to admit that my marriage ended because of cheating.”

Miley's still chilling with Kaitlynn -- having attended the MTV Video Music Awards with her this week while Kait's ex Brody Jenner's been dating Josie Conseco, daughter of baseball Hall of Famer Jose Conseco.

Hey, people move on, right?!