My own personal fanon dictates that such a thing would have happened somewhere in the SS arc or before the HM arc, one night after they'd snuck out of his house to dispatch a rogue Hollow.

On their way back inside after the fight, which had been bloody and violent, Ichigo reaches his hand to her cheek to wipe away a smear of blood. Before she can even smack his hand away in mild annoyance, she sees the look in his eyes as he does it, and she impulsively leans into his hand - causing the teenaged Ichigo temporary lightheadedness due to blood loss as he sports the hardest and fastest erection of his young life.

^____^