Because the MC that is not on the Jill path is never tested. Only the players who play multiple or all paths know about it so that the MC has been tested negative.
But that doesn't mean it has to be that way for an "Others", Isabella, Sage or MJ single run.
The danger in playing multiple paths is for the player to mix them up. What is true for Jill is not necessarily so for MJ.
For MJ, for example, there is a different hedge. In EP9, it is implied that Josy and Maya had only one relationship before the MC, their own. In threesome sex, we learn so that Josy and Maya have never experimented with anal sex. If the MC was faithful, he only fucked Zoey without a condom before MJ. That pretty much limits an STD and if it did, it would be easy to track. Unless Zoey fucked another guy before she left for San Diego. That's even implied in "Interlude." However, over a year has passed by then and the MC would have shown symptoms if he had an STD.
But there are also diseases, like herpes, that are transmitted through sex, kissing, or general contact and never go away because the virus stays in the body forever. I don't think so that the MC on the Jill path was tested for herpes. However, you can find "herpes" on the English mini-game. Maybe this is just a coincidence, a hint or a joke from DPC. I only know, so that in EP7 DPC lets the MC explicitly think something. Namely, the MC stands out so that Caleb has something on his face. What did Caleb probably do? He made out with Arieth in EP5. What is the MC prevented from doing in EP4, thank goodness? Kissing Arieth while "spin the dildo."
View attachment 2420125
However, Elena and John have sex with Arieth, which we learn in EP6. In EP8, the MC can then have the special shot with Elena, but it is harmless because the alcohol disinfects.
But I think so that in the Throuple herpes can occur and could cause trouble. The triggering reason for that will be Jade. It doesn't matter if the MC fucks with Jade or not. The screenshots show the transfer from Jade to Tybalt, through a harmless gesture.
Since this herpes virus could be spread in the Burke family, Josy can get herpes through Sage when she kisses Josy for fun in EP9. Again, harmless. But the harmless disappears if Josy, Maya or the MC gets herpes and another person also gets it at the same time. This could then lead to misunderstandings regarding fidelity.
Fictitious example: The MC goes with Maya and Josy to the gym and Josy has a herpes outbreak. Stupidly, at the same time, Caleb is also affected, but the MC and Maya are not. This can lead to a misunderstanding.