I'm lowkey confused by this comment, but let my try to answer:
1) ML does actually benefit a lot from having sex with MC, since MC has plenty of yang energy that helps ML, so i suppose you could say that there is a need for it
2) he actually fr thinks that MC earned his keep by being a prostitue. He is rude, but he calls it what he thinks it is, hence the whole whore-namecalling. Remember, MC was thrown out as a kid, but was earning money through cheating at mahjong only for a short time. He is hiding the fact that he cozied up with the beggars, so ML drew the conclusions that MC must have earned money through prostitution. MC has never once denied being a prostitute, or any accusation of having slept with multiple other men or wanting to sleep with others.
ML apparently does not give a fuck about MC (supposedly) being a prostitute, as long as they can successfully exchange yin and yang energy, but he gets annoyed when MC does not perform up to the standard that an experienced prostitute should have, so he starts making comments on it. He doesn't actually know, that MC only did it with a woman previously (and I strongly suspect that that beggar leader woman was his only previous partner before ML). And yeah, he is the ruthless leader of the demonic clan, dealing with someone he thinks is way beneath him. He isn't going to be nice and use flowery language.
I mean, it's not nice, I'm not excusing his behaviour, but it's kinda what you could expect from him, lol.
Why is ML being such an asshole like is there any need to put his oh-so-glorious sausage inside MC's hole? Because he keeps calling MC a whore, in the most disgusting ways he can think of （￣へ￣）