Do 男人 and 女人 have an unfavourable connotation? I've sometimes earned laughter when I used 一个女人 or 一个男人. Does it have a derogative connotation?

Is it better to use 女的/男的 or 女生/男生? Are there regional differences in the usage?
男人 and 女人 are used when talking about the relationship between women and men or when talking about specific characteristics of 男人 or 女人. Without knowing your detailed context, my best guess would be that you use them to indicate a serious situation where the generic gender terms should be used, which are 男的 and 女的, or just 男 and 女.