nerdDear artist: The chainmail is drawn wrong again... He's most likely doing European 4-in-1, which is both the easiest (generally) for beginners, and iconic.
The 4 side rings should not cross each other, only the center ring.
In the event that he isn't doing European 4-in-1, well... It's no 4-in-1 I've ever seen.
/s**nerd
my assumption was that she poked his crotch with her foot/brushed against it or something similar and it happened because his lust has been steadily growing but they haven't acted on it yet. she lives in the house almost all the time and has no tolerance or requisite knowledge for lewd stuff due to being kinda sheltered 1200 yrs ago, he might not be tending to his own needs out of respect for herWell, so it's seems that we have two options here:
1) he really is a fast one
2) the manga skipped what happened