The MC has no reason to lie about anything especially since you're supposed to pick the "truth" dialogue. If you refuse to take it for granted what the character clearly says, then you can refute any single lines of the game. You can also refute the fact he was telling the truth when he says that he had no girlfriend then. But for some reasons you don't here.
What's the point for him to lie exactly ? Having sex with only one girl in the past doesn't make him a Casanova, or doesn't make him look tough.
If the main character had sex ( complete and full sex ) with Cassandra of Jenni and if you pick
the tell the truth option he will tell the truth to Eve and mentions that he has sex just a few hours ago. That's something factual. If you had sex with both Cassandra and Jenni, he will say two instead of three. So, if he's saying the truth here, why exactly would he lie to Cassandra and Eve about the fact he had sex with a girl months ago ?
You haven't seen anything because apparently you don't care and you refuse to believe what the character is saying for some unknowns reasons, even tho there's no point of lying for something so insignificant.