Girl or boy thing?
I would only say the girl thing matters if you are wearing a dress. Otherwise, it is easy to lift a short/jumper/t-shirt and inject regardless of gender.
Dropping your jeans or injecting through them?
Depends high you wear your jeans and how low you inject.
Unless you keep your nipples warm with your jeans and inject in your unmentionables, what is wring with lifting your short or prising it apart between a couple of bottom buttons?