One possible theme could be sexual awakening. If it's a story about a character's journey, in part 2 they might be experiencing new aspects of their sexuality.
It might also have a theme related to the impact of external factors on sex. Such as how cultural, social or technological changes affect sexual behavior or attitudes. This could be explored in 'changes pt 2 sex story' where the 'changes' refer to these external factors and their consequences on the sexual aspects of the characters' lives.
I'm not sure specifically what 'changes pt 2 sex story' is about without more context. It could potentially be about changes in a relationship related to gender or sexual aspects in a second part of a story, but it's really hard to say for sure.
Another theme might be trust. For example, in a relationship, if one partner gives the other a free pass, it shows a certain level of trust (or a lack thereof, depending on how it's presented in the story). Also, it could touch on the theme of jealousy and how it affects the relationship when a free pass is involved.
One possible theme could be self - discovery. The bikini might be a trigger for the character to start exploring their true self in terms of sex and gender.
Maybe the theme of acceptance. Whether the society in Sun City accepts different sexes and how people deal with that. It could also involve the theme of change, like how the concept of sex has changed over time in Sun City, if there are any historical or cultural shifts related to it.
One possible theme could be growth. If it's about a young dog's first time with the shepherd, it might be about the dog growing and learning under the shepherd's guidance.
It could be a story of adventure. Maybe it involves a group of people embarking on a quest at that time, whether it's in the year 1444 or related to a time marked as 1444 in some other way.
I'm sorry, as I don't know the 'derdian new era pt story', I can't provide its key elements.
If it's about a mom and a girl (assuming Girlferand is a girl), possible themes could be love and family. The story might focus on the mother - daughter relationship, showing how they support and care for each other.
Possible themes could be time management, as it's a story within six days. Another could be personal growth, if the character in the story experiences changes during those six days.