That was why I mentioned that having Josy and Tommy having a sexual past together would piss of the anti-NTR crowd, something that DPC is extremely reluctant to do. We have the prostitutes who could conceivably not fuck their clients (we do have confirmation that they do oral (Rusty mentions Rio gives great blowjobs, Cam herself tells MC that she gives clients blowjobs but doesn't have sex with them)) we're told that Rio "hooked up" with Jamie but that phrase is ambiguous and can mean anything up to and including intercourse. Sarah and Mel more than likely do fuck their clients (and probably other guys without being paid) but we don't see anything to confirm that.
The only girls we are specifically shown to have fucked other guys (or we are explicitly told have) are Arieth (not fuckable by MC), Becky (not fuckable by MC (yet) but blowjob possible), Tania (not fuckable by MC), Heather (not fuckable by MC (yet) but blowjob possible) and Elena (not fuckable by MC (yet, if it is then it would probably involve the guy who has fucked her either watching or joining).
The only fuckable character who is heavily hinted to have fucked another character is Quinn with Vinny suggesting he fucked her in the past, but those people who are anti-NTR can just ignore that and believe that Vinny only said it to wind Quinn up and he is lying about it.
It seems DPC doesn't view oral sex as counting as proper sex, so he's happy for side girls to be confirmed to be giving blowjobs to other guys, or for MC to receive blowjobs from characters who are confirmed to have fucked other guys, but he won't have MC fucking a character confirmed to have fucked another character.