hi! i read this for about 5 times for the past 5 years, and i could tell you... it's one the best out there. im not even kidding.
to answer ur question tho, no, the sleeping with others lasted for like less than 30 ch iirc (i dont really look at the chapters), so yes, it's just for the "beginning plot" and there's no SA! they actually have a healthy (i would say) relationship between them (if only they weren't living in joseon era when having mlm/wlw relationships r so forbidden they could actually end up dying )
it's still famous even after ending for years, and you'll understand the hype once u actually gave it a try!
I'd argue Beom is an unrealistic character which is why there is no SA. It's implicated that he forced himself to sleep with others to survive and he seems negatively affected by this. But because the bottom tells him to, he seems ok with sleeping with all these people? Technically there is a power imbalance that means he can't really say no. Like, it doesn't add up to me.

I don’t like reading about the mc or ml sleeping around with others but everyone keeps praising this story like crazy on other platforms. I wanted to know what it’s like before I pick it up. It it only a beginning plot or is it throughout the whole story that they basically sleep around with others? Cause that’s a deal breaker (also SA plotlines, I would like to know if this story has it so I can drop it now)