Does the "bloody nose" trope necessarily imply a sexual situation?
Gilles Portras, author of The Anime Companion, wondered the same thing: 


  "So I asked a few Japanese and got a variety of pseudoscientific, and occasionally embarassed, explanations about humidity and blood pressure. But the best response I got was from one fellow who simply recounted that when he was a child he was told by his mother that if he stared at a pretty woman he would get a bloody nose."


It's basically superstition, like if you sneeze once someone is saying something good about you. If you sneeze twice they are saying something bad. If you sneeze three times, you have a cold.