Nope. never happened. You're delusional.
LaLaLaLaLaLaLaLaLa.
IF it happened, MC was passed out drunk and Mia raped him. Maybe, Mia just crawled into his bed and started playing with his dick. It wouldn't be the first time.
Seriously though. While that hints at them having sex, it could also just be saying that this isn't the first time Mia has tried to climb out of the friendzone and MC had to put her back in there.
All we know is that MC knows he had sex and it wasn't memorable. I can't see how he can know he had sex, but not know who with (unless it was just a random). If it wasn't a random, then he has no memory of having sex. So, who told him he had sex? Only the person he had sex with or it was public. So, if it was Mia, he would have to known. It's also pretty shitty to say it was crap while Mia is in room. I think it's a red herring type of clue.
Then, that intimate scene that actually came out of nowhere and didn't fit the story at all, if you weren't following Mia path. On Mia path, it was probably perfect. But on a Sophie or a Bree and turning Mia down, you have pushed her away constantly and suddenly she's climbing all over MC on the lounge and he just cuddles her. Then you get another chance to blow her off, and next thing she's crawling into bed and playing with MC dick and he just let's her. If you're not on Mia path MC behavior is really weird.
If they haven't been intimate before (or they have and he doesn't remember), why is he letting her play with his dick (if not on her path)? If he does remember they've been intimate, why does he say he doesn't remember the last time? No matter what, if you are not on Mia path, that is a really weird scene that doesn't fit the previous gameplay. Or perhaps it's just bad dialogue pathing/linking and I'm reading too much into it. I do that sometimes!