I would tweak that slightly: the MC is unlikely to get intimate with any female character WE SEE having sex with a male NPC. We know that Chad and Sage must have had sex at some point in their relationship (NTR haters might want to pretend Chad could never get it up from day one if they want to, but that's just not believable), we know Tommy and Heather are doing it, we know Camila, Riona, Quinn, Sarah, and Melanie "serve food in the restaurant," etc. etc. Not to mention Nicole, Sandy, and the other Pink Rose girls are obviously seeing lots of customers. It's true we've never SEEN them having sex with other dudes, but it's obviously happening. The only quasi-exception to this might be Elena and John Boy. Assuming we have a chance with Elena at some point, which seems possible given how heavily she's been flirting with us, I think we saw Elena and John Boy make out in episode 4. Oh, and of course we see Maya and Josy have sex with each other. In short, the delicate souls who just can't tolerate anyone else having sex but them will either need to contort their imaginations into fantastical pretzels, get therapy, or go play something else.
Most of the characters are sexually experienced (with the exception of Jill, and Maya when it comes to boys), but regarding Sage and Chad, the jury is still out on that one. When Chad comes to her to ask for help with his being kicked out of the Alphas, she says to him:
Sage: "Oh! And it's about fucking time you start getting more physical with me, too!"
"About fucking time" sounds like she's pretty pissed off with the physical aspect of the relationship.
Unless something is clarified regarding Chad's sexuality, I kinda get the impression he is totally gay, and just used her as a beard (probably because she was coming on strong to him so he took advantage). How long they were in a sexless relationship is unknown, it could be months, or years.
The flashback where Sage is asking Chad about Halloween preparations is about 2 years prior to the current events, but we don't know if they were in a relationship then (he was certainly ignoring her though).
So yeah, not sure about that couple, but just about everyone else is, or was, fucking someone else at some point. But yeah, as soon as we see someone with a dick in them, they're suddenly off the mc's menu.
Heather would be the closest thing to breaking that rule since we see her lying in Tommy's bed (potentially with another girl) right after the deed, but I think she is the exception precisely because she's Tommy's girl.
But yeah, the sensitive types can all believe that:
- Bella's husband died before they had sex
- Jade adopted kids because she didn't want to have sex with Steven
- Sage, Riona, Quinn, Cammy, Mel, Sarah, et al. are all virgins who have been practicing with their pillows.
- But KRJ is a total cum slut!