As far as we know, No. Everything points to Joowon ever being interested in Haesoo and no one else.
yes the author answered it in a q&a
Ohh... Allright. Ty
Q: I'm curious about whether Joowon had any past relationships before this.
A: It seems as though it wasn't reflected well in the webtoon, but Joowon has dated a lot of people. Haesoo tends not to hide his relationships from Joowon, while Joowon tends not to talk about them.
In the beginning, it can be seen that both of them talk about Haesoo's relationships comfortably, and they talk about everyday affairs too.
So I Wana know whether JOOWON fucked anyone else other than Haesoo?? I don't remember the story well,.. Did he??.help me??.