What Could Have Happened Between Kazuya and Jun in Tekken 2?
Tekken presents a rather ambiguous background between Kazuya Mishima and Jun Kazama which gives birth to their only son, Jin Kazama who looks a lot like Kazuya Mishima. So what we know based on canon is that they met in Tekken 2 and in Tekken 3 which happens 15 years later after the second game. Jin steps in revealing himself as Kazuya's son therefore Heihachi Mishima's grandson.
The only possible thing I can imagine is rape rather than consent because I doubt it Jun will ever fall for Kazuya in a million years. Namco hasn't really released an official word on that but with its T rating, a rape plot can always be added without making it obvious or detailed. I wouldn't think that Jun Kazama a WWWC officer would even consent to Kazuya so I thought, maybe in the middle of the second tournament or whatever, Kazuya may have banged her against her will (and I don't think her ending is canon either). She could love Jin, I mean some women who became single mothers because of rape or a cheating boyfriend can love the child that was born from it. In the process, the Kazama blood weakened Kazuya therefore making him vulnerable to his insane father Heihachi.