I mean, putting aside I was half-joking, this was more because of how we have seen Suguha specifically prickly with Sinon, the way she was looking at her IRL when she tried flirting with Kirito in the bar was quite telling, AND if you recall last time she actually actively PREVENTED Sinon when she wanted to escalate their 'competition' to actual vaginal sex because of, quoting "Even I haven't done that yet", because she wanted it to be special. Reminder, again, as much as I love Suguha's plot in this game, I won't pretend Fujino Suguha makes sense or is particularly mentally stable, if anything it's the opposite, and her reactions can vary quite particularly, like, she was basically about to turn into the Incredible Hulk out of rage the more she 'scanned' the house and saw what Asuna had been doing behind her Onii-chan's back, but because Fujino can't let her just "win" she didn't explode and reveal the truth at him despite how realistically she should have for a while now XP