Maybe it has something to do with her, I don't know, evolving as a character over the chapters? You know, she experiences things and gets bolder and more corrupted as she does so, indicated by the flower petals changing from pink to black on her portrait. As it has been pointed out to you many times, she made her own decisions completely independent of F in Chapter 1 and 2, so her making choices in Chapter 3 should hardly be surprising. The first choice she makes is actually on the first day of Chapter 1. She can go to the party or take a shower. You, the player choose what she does, that makes her one of two playable characters. She also make the decision to start a sexual path with Elena in Chapter 1. She has always been co-MC. I grant you she didn't make many by herself in Chapter 1 and 2 but that was because her character was still fairly sheltered.
Chapter 3 is her liberated from her sheltered life and instead of being surrounded by people wanting to hold her back, she is surrounded by people wanting to help her flourish. Her self confidence is building all the time and she is starting to think about what she wants and is tempted to act on it. Expecting her to behave in Chapter 3 like she does in Chapter 1 and 2 would be to ignore character progression. Heck, D in Chapter 2 is different to D in Chapter 1. There is no way Chapter 1 D would have stood up to her mother the way she did.
As for sharing with guys, well, she is already used to sharing with other women by Chapter 3 (assuming you did those paths), it is only natural to become curious. Especially when the path only develops if she watches porn with two guys and a girl, a choice you make. Your dismissal of the male sharing path feeling unnatural just shows you don't know the story on that path and what has led her to these. It has been a slow burn and feels like a natural progression, if not doing a solo run.