lets see... mentally competent adults in private. That's the generalisation. There are dozens of sensible nuances, modified by the golden rule trying to take care of each individual feeling and taste, and balancing each person's needs against other people's protections.
Outdoors is fine, if you can manage not to be caught by kids or other adults that would be offended.
Watching other adults is fine, or being watched, if you join a club where that's part of the point.
Having sex with multiple partners, ditto. Private houses instead of clubs, where there are party rules, allowed.
Swinging, ditto.
Sado-masochism and bondage, fine, as long as there is both consent and safety measures put in place, and safe-words that are strictly adhered to.
Domination and Submission, ditto.
Teenagers unfolding their sexuality and having sex with other teenagers, fine. The law in Aus is that they are supposed to be within two years old of each other, to try and minimise maturity and personal power differences between them. I'm not opposed to them being more than two years apart, if each one is confident about saying no when they are uncomfortable, and each one is attentive and will stop for the within other's comfort.
Little kids playing "I'll show me yours if you show me mine" together, fine.
Little kids spying on adults occasionally out of curiosity. Fine, very educational.
Mentally ill people are usually compos mentis enough at least to handle their own sexual relations. If they aren't, that's a whole other medical issue for the person.
Mentally retarded people kind of need care and shelter. For each individual, their unique situation, in which hopefully a guardian will give them a caring and safe space in which they can explore and enjoy sex with people they are mutually interested in, and as much privacy is granted to them for sex as is commensurate with their safety. Their guardians must think through and help handle their contraceptive issues, and must be alert that they are not taken advantage of or molested.
Permanent group relationships, fine. They don't have legal recognition in the west yet. So far, not much of a public legal issue yet. Perhaps there's not many with the maturity or inclination to want one. Perhaps almost all who do participate are the sort of people who think marriage is anachronistic anyway, and would be happy as defacto partners to a sole person, as well as to more than one partner. In Australia you cannot construct a legal polygamy, but you can keep your polygamous marriage if it preceded your residence here.
Group marriages from other cultures, fine, as long as the power difference between all the members is not too unequal. Each member should be a consenting adult, someone who really wants the relationship that way, and who has the power and resources to walk away from the situation if unhappy. Western culture has no room to throw stones at group marriages where the females are tyrannised into them, and within them, since this used to be a characteristic of the majority of our marriages in our recent past, historically speaking.
An over-age partner with an under-age partner: they shouldn't be too many years separated in age, for power difference reasons. In this situation, it's a matter of context and the individuals involved. I wouldn't raise my eyebrows at an 18 year old with a 25 year old, or a 17 year old with a 23 year old. A 16 year old with a 27 year old is getting questionable. Individuals develop maturity at different rates. Some teenage girls and boys are very mature and independent, with advanced life skills, perhaps already earning a good income. Some of them are immature, mentally/emotionally ill, hooked on drugs, whatever. The former deserve their sexual independence, the latter need to be protected from being preyed on.
An adult or teen trying to have sex with a pre-teen. Well that's not on is it? Do you really want the exposition?
As a summation, it's all about making sure sex is about pleasure, and not about pain. Unless pain is erotic for you and wanted and you are competent to involve pain in your life in an enriching way, and not a destructive way.
