I believe after Nabi leaves for her aunt's place he does sleep with the woman at the bar - that time he was still sorting out his newly realised feelings for Nabi but after he talks with his mum and decides Nabi is the one, you don't see any of that anymore.
I think it's pretty clear that Jaeeon was with plenty of women, which is why he was very clear that he didn't want to commit to Nabi when they first hooked up (contrary to baffling general opinion I keep reading on MDL, not from you from others, he did NOT cheat on her).
But why don't they show us clearly if he sleeps with other women when he starts with Nabi? For one it's to show that maybe him sleeping with a lot of women is an exaggeration by people around him, to an extent. Maybe it's the gossip that Jaeeon is always subjected to but never protests. If you watched Ep 10 when the friend group who always talks about him admit that they don't really know him at all.
