Yes, but you have the opportunity to deny it, which you don't if you have slept around. Jamie also replies to Becky asking if you're single on rooster (after helping Jill with concert) that you've always got something going on.
I don't think DPC considered anyone doing a celibate run. However it should be noted that the DIKs have seen MC being dry humped by Sage, Josy coming out of his room early morning after sleeping over, possibly Bella coming out of his room after sleeping over even on a celibate run so the reputation could still come from that.
i havent slept with either of those. plus the deal to get a slap in front of diks also happens if ya only helpout with the underwear. without having sex with em.
Bella only sleeps over if ya helped her fences and are in a semi relation with her.
as for the rest, Sage also says that you shared some intimate details about ur birthday, even when ya let her sleep and dont have the conversation with her.
as for Josy, yes even though she only sleeps with ya (Without sex) the diks still asume ya had sex with her.
my latest playthrough was the totaly single route, no sleeping, no sex, no kissing, no flirting, only 2 or 3 minor dik choises, the rest chick choises (Major and minor)
so hence i know whats what (and yeah even then the pregnancy test showed, thats why im asuming its either tight to events in our mom's diary, or DPC asumed noone would go for the single life on one of their playthroughs.