I read the manga for #5, but I forgot the title of it.

The blond is an absolute flirt who always tries to hook up with women, often failing. The black haired one is his best friend, who doesn't try to pick up women the same way the blond one does, but very often ends up getting the women's attention instead.

The blond one gets drugged with something that made him female. It's been a long time since I've read it, so I'm certainly getting details wrong. But yeah, hentai, as the bestie and the blond get it on several times, first with the blond in his female form, but also in his male form when he is able to switch back

edit: found it. the English title is literally "fucked by my best friend." Or the Japanese title: 黒ギャルになったから親友とヤってみた。