Asuming he is my son , is him being sarcastic about it...
Since he already knows 'he' is not his son.
But its just he does not have proof right now.
Also that lady blonde, even fmc find it sus. That why is she so threatened by fmc , cuz as u know she is supposed to be the 'favourite'.
So ml is not confirmed slut now , but neither is totally confirmed till this point that he had been vompletely loyal to her.
I said that in my comment tho him saying "assuming he is my son" means hes definitely slept with her, he just knows that hes not the only person to be sleeping with blondie so the kids lineage is questionable. It would also explain why blondie is so scared, they're definitely sleeping together but she knows its only temporary since he probably explained it like he did to the 1st concubine that the only woman who'll be his wife is FL and shell only ever be a concubine, and now that hes sleeping with legal wife she'll lose her position (as his sexual partner) and favor (though he probably never had any and was just using her for sex). If he knew for a fact the kid wasnt his he would've definitely gotten rid of the blondie.
Also there'd be no reason for him to stay for 3 nights with blondie other than sex seeing as he doesnt particularly like "his" son
Your point of view is also valid, if its true
I will not read this further.
But i am going to give this story, benefit of doubt and not assume the worst of ml.
But if in any future chapter , what you said becomes even slightly true. I am dropping this.
Sick and tired of ml , who are supposed to be written as yearner but never actually show any of the yearning.... [show not tell].
So much for , i have been holding back. What about this is holdig back is not he gets his sexual and emotional needs met with his concubines what was he holding back?
Shit??
But i am also 50% sure thats its all just a misu derstanding and he was really a yearner...if someone has spoiler , spoil me pls.(๑•ㅂ•)و✧

"Assuming hes my son that is"= "I've been hitting that but I probably wasnt the only one she's slept with so theres a chance hes not my kid"
"He's at most spent 3 nights consecutively
with lady hong"= he at times hits it for days on end
Ml is a slut confirmed, but its probably gonna be some BS like hes always loved FL (bullshit) but he didnt want to touch her as he forced her into the marriage but he only did so to save her from being executed alongside her family but now that she wants him he'll graciously stop sleeping with his 5 other wives that he forced himself to sleep with so they wouldnt bother FL because he's pure hearted now, until they argue or some shit and he runs off again idk.