I don’t really believe in soulmates so i think they were compatible people who bonded in a difficult time… yj desperately wanted to get to Choose who he spent his life with, rather than be given an assigned match, and mona wanted someone who made her feel safe and desired in this scary new country… don’t forget she’s 18 and has only been away from home for less than ONE WEEK on the night she finds out about soogyu and first turns to yj for support… probably away for like a grand total of a month when yj knocks her up… she’s in a really lonely tricky spot and she’s just lucky that the person she turns to is a genuinely good man who happens to also be lonely and looking for connection and thus is thrilled to take her in and give her the love she’s craving. I think YJ is also more pragmatic than soobin about emotions and mona plays right into that pragmatism … soobin is a dreamer and is able to make a completely impractical love story come true just by his sheer refusal to think it’s impossible, but yj struggles to even envision falling in love with someone and getting to have them when he’s been set up to follow the typical path of the crown prince his whole life. so i think when he finally breaks the rules and has his affair with mona, the convenience factor isn’t irrelevant to him lol… she is definitely the easiest person for him to fall for and even in a world where she is soobin’s wife forever yj knows soobin won’t be upset at all about them being together and would in fact happily cover for them as long as they do the same for him and gyu. Yeah the whole situation is definitely more right place right time but at the end of the day i don’t believe that means she’s NOT the one for him… after the war, i doubt yj would ever want to be with someone who hasn’t been through that trauma with him (remember it wasn’t exactly easy for him back at the castle strategizing for battles that might kill his baby brother), and mona was his rock the whole time and absolutely won his heart through it. I think the two of them are very happy together for the rest of their lives and have 3 babies and once yj is king he legitimizes all of them (and “adopts” the one that’s SUPPOSEDLY soobin’s) and the kingdom loves him so his Sexual Indiscretions are just kind of just not a big deal and people think their love story is so beautiful etc etc