After the courtship was announced, Snape got challenged everywhere he went. It was ridiculous. Most people weren't even proficient at dueling. Then it became clear the experts had just been attempting to let the masses wear him down. Snape started fighting major duels daily, sometimes several times a day, instead of the previous quick disarming.
Wizards and witches were coming out of the woodwork, sneaking into the school - using polyjuice, old secret passages no one sealed up, even after Sirius and Pettigrew and even Draco. (Harry wondered if Snape and Mcgonagall even tried adjusting the wards or if they hadn't had time.)
The first few duels Harry felt obligated to watch out of guilt. Snape was only courting him to protect him after all.
But after watching those duels and seeing Snape absolutely dominate them, Harry started to watch for other reasons. He flushed at every precise wand movement, thrilled at every quick movement, almost too fast to follow.
By the time the real duels started in ernest, Harry was just soo turned on watching Snape. He could hardly bear it.
How dare Snape be so competent?! How dare he smirk like that. Hold his wand with such strong but elegant fingers. How dare he look amazing and confident and so fucking determined on Harry's behalf, even when he was covered in soot and bleeding (just a little).
And how dare the current challenger manage to tear Snape's shirt? Harry got a wide-eyed glimpse of chest hair and a nipple and felt like he was having a heart attack. Not to mention what was going on in his pants.
What even was Harry's life?!