In order to advance the main quest, you have to go to the locations in the map linked on the first page (NOT the starting out guide, the other one). Otherwise, this is a survival/resource gathering game; gather resources to make stuff, and eventually you'll be able to make better stuff.
Nearly every unique creature/human in the game can be screwed. You just have to choose the right options when interacting with them. Bigfoot and the werewolf and most male bosses will rape the female mc if you screw up fighting them; the male mc can recruit such as the giantess or the big-tiddy lady and fuck them. For screwing the locals, you have a couple of choices: 1) get defeated in battle by male npc's as the female mc or attack female npc's as the male mc, 2) beat whichever native you'd like to bang into submission with a club (not the spear or sword, ONLY blunt weapons), put the beaten person into your inventory, build a cage and put them in it, and feet them cooked meat until their little icon says "maxed", THEN explore the island with them and/or dance with them until their disposition towards you reaches 51. Then you can ask them for sex. You can build a little colony this way, and you can even assign the abducted npc's to do stuff such as harvesting wood or mining.
Btw, if you explore the parts of the island that are for the main quests, you can even see the icons on the map. Blue diamond icon is for the male mc, red/pink (I really can't tell) one is for the female mc. You have to travel east/northeast pretty far from the beach to find the first main quest. Here's a little advice for you that (hopefully) doesn't spoil much: craft as many red flags as possible; wherever you place them becomes a fast travel point to help you get around the island a little quicker.