You get a lot of red flag behaviour, because fanfiction authors usually love to take the short route to the sexual tension they love.
The characters are often mere projection surface.
Your average female lead usually is young, slim, has a beautiful face and nice bre**** (this is emphasized a lot). She's said to be smart, but at the same time naive, she needs her knight to guard and guide her, she's clumsy which is often borderline stupid (and defies the smart part), but you should view it as cute and it will make the knight shine some more.
Your average male lead is older than her, but usually not more than 10 years, hot, slim but muscular, higher in status and wealthy. He's magically infatuated with her and will do anything to be at her side, his mind is set on her alone and he will know what to do. In the end he safes the day and all he wants as a reward is her. He will pull her in his strong arms, press her against his broad, firm chest and give her the steamy hot kiss the audience craves. All the better if he pushes her against a wall or in the sheets.
See how easily that can turn into red flag behaviour?