(guy) I don't get it.
Jun did just say he was now reluctant to hurt her with his hands/fists. That'll make sparing a lot less fun. Plus now he doesn't have the motivation of following her strong back. It's different, so there will be an adjustment.
To be fair/even-handed, there are plenty of competitive guys who start to lose interest in a gal as soon as she's "caught" (whether that means agreeing to date, taken him to bed, or just feels bonded). You know: she seems to look better running away with a flirty look over her shoulder than gazing up lovingly from right beside you.