When reading the vedic scriptures or hearing about it, one may come across the topic of illicit sexuality sooner or later and about the many things that are not allowed. For example sex only in marriage and other humanly invented systems.
I want to simplify the whole topic a bit, by going to the root issue. Think about it logically, sex in nature is there to produce offspring, ideally good offspring, thats the materialistic side, in the vedas there is also the spiritual side, where the goal is not just to produce good offspring, but also help enable them to make spiritual progress and ideally reach enlightenment or salvation, whatever you want to call it. That is all what sexuality is for and should be used for and everything else is illicit sexuality. All other rules that may have been created in human society regarding that are just there to ensure the primary goal is met and to save human society from so called undesired children, which are probably the greatest cause of disturbance in society.