fuck literally anything and everything you can. you gain it by doing any and all sex scenes, I use the troll from the bridge scene when he has high like to farm for butt and mouth experience and it works pretty well! so I suggest you try to find a character scene where MC is top and is repeatable anytime and as many times as you want. if you're ok with overleveling your appearance stat though you can use that and insta win fights against the satyrs who you can face or butt fuck after getting to max lust in the fights. if you already have Everett and Feliar fucking you can also do the sex scene with those 2 and choose to either top or be in the middle to increase your cock stat too. plus as VolxdoSioda said, you can just jerk off too. if you get the sexually exhausted stat from fucking too much you can just throw lust potions I believe.
Thanks for the replies
I want to ask another thing. When finishing the Lizard treasure quest, there are "make a speech" option and "do your best" option when sitting between Othra and Roushk. But in my case there's only "do your best" option. Is there something I need to do to trigger the "make a speech" option ?