Most likely...

Sarah whispers in your ear “So how does fresh real semen feel flooding your womb?” You grumble still full of lust and not quite aware of reality yet. Your sister chimes in “Ya, we finally got to use some of your brother’s batter on a willing ‘victim’. It’s nice of him to leave full condoms lying around all the time after doing his girlfriend. Something finally clicked in your head and involuntarily scream “Oh my God...sis what did you do? I’m gonna be stuck as a girl...”