Actually the flashback doesn't show them together. It shows Sage go after Chad and Chad ignoring her. They could be in a relationship, but it could be that Sage was crushing on him and trying to make a relationship happen. They might only have been in a relationship for a few months at the start of the game.
Ok, you are right.
I didn't load up the scene but I found the dialog in the script.
All it shows is Sage talking with Randy and Tommy as they are setting up the DIKs.
Then Chad comes over and Sage says Hi.
She confirms that the HOTs party with the tri-alphas and asks Chad when they are going to start planning for the years parties.
(This was her first year as the HOT president so she needed to work with him on party planning.)
Then he leaves.
So I totally read too much into that.
Given all that, there isn't anything in the game I can think of that points to them being together for any long amount of time.
Hell, it might just have started this new school year.
I now think that they have not had sex at all, given this info and the things she says during their Ep3 fight.
sa "Hi, Chad!"
scene ep5_intro_part4_20 with dissolve
ch "You two need to get familiar with how things work around here."
ch "No one parties with the HOTs, but the tri-alphas. That's how it's been and how it always will be."
ch "Right, Sage?"
scene ep5_intro_part4_19 with dissolve
sa "Yeah."
sa "Chad? Do you have time to plan for this semester's parties?"
scene ep5_intro_part4_24 with dissolve
ch "I'm busy."
sa "Wait! I have some great ideas for Halloween!"