Uh... the main source of income is definitely sex. I don't know if that was intended to be the main source of income, but it's far and away the easiest.
And, well, what I was missing is not losing Mental by clicking to go places most of the time, because nearly all of the random events cause mental loss if you aren't wearing clothes.
I think I have some confusion regarding the goal of this game... Especially if you weren't expecting players to acquire income primarily via sex.