Premise:
Here, public sex and nudity for people of all ages and genders are not just completely legal, but are extremely common and part of normal life. People are bisexual or pansexual, always horny, and there are very few sexual taboos. As a result, for all of human history, societies have been free use, with minor differences from culture to culture.
In the modern day free use has been codified as international law, so people can have sex freely regardless of where they travel, and has been in place so long people cannot imagine things any other way.
Worldbuilding Notes:
Sexually transmitted diseases do not exist.
Menstruation does not exist.
The anus is for sex only. All bodily waste is output as regular piss. Toilets do not exist at all.
Anal sex is as safe, clean and easy as vaginal sex.
Everyone has conscious control over their fertility. Unwanted pregnanices are impossible, both partners must want it. Consequently, condoms do not exist at all.
The weather everywhere is usually warm and comfortable year round.
Humans have very short refractory periods, and can orgasm again within seconds of a prior orgasm.
Humans grow very little body hair. Most people are smooth, with a handful of people growing small bushes or landing strips.
Humans produce significantly more sexual fluids.
The average penis spurts out 70-120 mL of semen, but medicine can make this much higher.
Penises are almost always dripping precum (1-3 mL per minute).
Vaginas are almost always wet, dripping constantly (4-17 mL per minute). It's obvious whenever someone with a vagina recently sat down somewhere.
People make huge messes all the time, constantly staining themselves, their clothes, and their surroundings.
"Sexual fluids" includes spit, semen, girl fluids, and piss.
People find the idea of "cleaning up after themselves" very unusual. You would never think to prevent a creampie from dripping out, or using a tissue to wipe a stray cumshot off the wall.
Sexual fluids dry up naturally over the course of 48 hours. Extra cleaning for sexual activity is unnecessary.
All electronics are water and cum-proof.
However people clean up non-sexual fluids. Dirt, grime, and food messes (except in a cafeteria) get cleaned up pretty quickly. Throwing random garbage on someone's bed would not be okay like ejaculating onto it would be.
There are no public indecency or lewdness laws. Sex in public is the default (there's sex furniture everywhere), and most people wear very revealing clothing or go fully/partially naked.
Sexual exclusivity does not exist, and because of free use requirements, cannot exist. All relationships are "open".
Most people are polyamorous and date multiple people at once.
Marriage can occur between any number of people between any ages or genders, not just one-on-one.
There is no age of consent, as soon as a child is ready and willing they become sexually active. This usually happens around kindergarten and is verified by a doctor.
The age 18 has no special significance at all.
When someone is said to "become legal", it means the doctor verified they are officially ready to have sex. However kids younger than also have lots of sex, this certification is a formality.
There is barely a distinction between "kids" and "adults", only a vague notion of life experience. Photos, videos, etc. of "kids" (even having sex) is just as legal and normal as for "adults".
There is absolutely no distinction, legal or social, between "SFW" and "NSFW".
PornHub does not exist, jack-off material is on YouTube along with any other videos. Besides most "regular" videos are almost pornographic anyways.
Social media is half friendly chatting, and half amateur pornography feed.
Incest is very normal and covered by free use. However some people still find it extra arousing because it feels "forbidden".
Beastiality is legal (as long as neither the human or animal are hurt) and is also covered by free use. But it is much more common in some areas such as in the countryside.
Watersports is also covered under free use, but is less common than cum.
The world is largely gender-equal. Femdom and maledom are seen as kinks, not as ways society should behave. Bathrooms, dorm rooms, etc. are never separated by gender.
There is no concept of "virginity".
Free Use Notes:
Casually having sex nearly constantly is seen as an innately human trait. Suppressing sexuality is unthinkable.
Sexual education starts in 1st grade and runs to 6th grade. In addition to anatomy it also teaches practical sex acts and requirements under the free use law.
Safety and health are paramount. If using someone would hurt them, you can't do it. If you can't fit it without pain, use enough lube or don't do it.
Hospitals are able to determine if patients are well enough to be freely used. If a person is deemed too sick, you cannot fuck them.
Dangerous jobs like mining or construction don't allow free use or nudity for safety reasons. However, workplaces must provide frequent "stress breaks".
Even though you have a right to use anyone's body (like they have the right to use yours), if they refuse or resist you, you can't force them. Free use non-compliance is handled by the police.
Likewise, sexual assault or non-consensual violence is not allowed. Such crimes are also handled by the police.
The following "basic acts" are always allowed. Acts not on this list (like hardcore BDSM) require real consent, not free use automatic consent.
Stripping someone, and taking their clothing (unless it's valuable)
Taking photographs or videos of them
Fondling or groping them
Masturbating near them, or making them masturbate themselves
Ejaculating on or in them
Kissing them
Eating their pussy
Sucking their cock or licking their balls
Using an approved sex toy on them
Having vaginal sex with them
Having anal sex with them
There's a lot of flexibility with what you can do as long as it's reasonable. For instance, since there is no effective limit to how many people you use, you could "use" a guy and a girl, and require them to 69 while you watch. You could order that cute girl to come to your room even if it's a little out of the way, as long as most people would consider it "reasonable". You can't use someone if it'll harm them, such as making them lose their job or not being able to attend class.