Is it allowed to talk openly about sex (sometimes about what happened in one's own bedroom or sometimes in someone else's experience) in a room where there are only females? Is it allowed to have fun talking about these topics? Please tell me with authentic hadiths if this is haram and worthy of punishment. It is a tradition to talk about sex in a fun manner during cousins' weddings nights in our village.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

There are some details about this; if a woman mentions what happens between her and her husband regarding the matters of bed and enjoyment, then this is forbidden. But we do not know any evidence about a specific punishment for this.

If the matter is just mere mention of sexual intercourse without mentioning any details, then this is disliked because it is contrary to decency, especially for women, as they should be modest and not get involved in such talk.

Abu Sa'eed Al-Khudri narrated that the Prophet said, "Among the most wicked of people in the Sight of Allah on the Day of Judgment is the man who has intimate relations with his wife and then divulges her secret."

An-Nawawi said when interpreting this hadeeth in Sharh Saheeh Muslim, "This hadeeth is evidence of the prohibition for a man to disclose what happens between him and his wife of matters of sexual enjoyment and describing the details of that and what the wife says or does during intercourse. As for the mere mention of sexual intercourse, then if there is no benefit in it and there is no need for it; it is disliked because it contradicts decency..." 

Allah knows best.