While it's not shown (that I remember... been a bit though), I'm pretty sure it's heavily implied that two of the main LIs do stuff together without the MC, given that they were in a relationship before even meeting him. But it's with implicit permission, so it's really more netorase (if that), and honestly better described as polyamory.
Adding on the the comment regarding your best friends wife, there's a whole planned side path involve her coming up sometime in the future. So definitely expect more of that type of content in the future.
Nice, the path with the best friends wife sounds great.