what is the practical use of some potions in the game, like gay/bi/str8/submissive/dominant? are they used in some quests or something?
Sub/Dom affects a lot of sex encounters, but it's a bit messy since they're also often tied to brave fame vs lewd fame. For example, if you're Dominant, you won't encounter the dominant stranger in the public hot springs even if your lewd fame is higher than your brave fame, but I'm pretty sure the fanboy only shows up in the hot springs if your brave fame is higher than your lewd fame. Meanwhile, if you Relax in the bar, the fanboy scene is all you get if your brave fame is equal or higher to your lewd fame, and your top/bottom/dom/sub stats don't matter. However, to get the drunkard scene I think you have to be Submissive in addition to a higher lewd fame (though I think the scene disappears once you make the drunkard a guard).
But generally, Dominant and Submissive players get different versions of some sex scenes, while Top/Bottom score does
almost nothing. Here are some scenes that change based on Dom/Sub score:
Tom/Andrey's scenes upon entering the city or watchtower.
The Outdoor Fun scene with the farmboy.
Sex with Jester.
Sex with Harold when renting a room in the Tavern (no love potion).
Seek Comfort and Love Potion with Feliar.
Helping with chores in the Northcrest Inn.
I'm pretty sure only Bottom players can have Gunnar show up while helping Bernard in the shop for the day, while Top players will never see him (one of the only cases where Top/Bottom matters at all); but only Submissive players can suck him off there, so you need to specifically be Sub+Bottom for that scene.
In Gunnar's Track/Bareshade content, Dominant playes will use the holes of the Novice Guard or Merchant in all of the caravan scenes, and never actually have sex with Gunnar himself (even if you whore yourself out in the tavern). In fact, most of the tavern whoring scenes change depending on whether your char is Dom/Sub; or specifically, whether your character is Dominant or not, since Vers players usually default to Submissive scenes.
On the Battlefield, Sub players with high orc relationship have 2 scenes where they can submit to orcs without combat, while Top/Vers players can help a Soldier in peril then top an orc with him.
Dom/Sub also changes minor text in a lot of scenes, such as in how you react to/interact with Gunnar, since he has more content for Submissive players (including during Combat with him).
It's actually a really annoying system, especially as a vers person who wants all the scenes (as setting yourself to vers/vers still locks out some content). As for Gay/Straight potions, I'm pretty sure those only affect enemy spawn rates (female bandits and lizard acolytes, for example, and maybe harpies).