I'm in my mid-'60's. By "category" what I consider acceptable:
1) BETWEEN CONSENTING ADULTS: (any form of sex, any number of participants): anything and everything that doesn't hurt (in the broadest, most inclusive sense) any of the participants.
2) BEASTIALITY: Never done any nor have any desire; but I have seen some porn and it doesn't strike me as doing any harm either to the person or the animal. Just this past week, I was told by a person from Colombia, that young men there use donkeys as a matter of course (what with the Catholic Church making young women as anti-sex as possible). This strikes me as a better alternative than either serious, long-term frustration, or frustration-driven aggressiveness towards others (especially unwilling partners). Overall, I still don't have a strong opinion either way.
3) CHILDREN (UNDER 16-18 YEARS OF AGE): I don't get my morals from some book ("holy" or otherwise). I try to think things through comparing harm vs good. I know that kids of all ages, on their own initiative, touch themselves, some masturbate. I see nothing wrong with that; how could there be if it is so evidently natural?
On the other hand, kids having sex with others seems to me to be harmful/dangerous to the kids. Why? Simply because sex tends to be so delicious, that it can easily be over-powering. Kids don't have the capacity to reason things out; and will tend to go "heads over heels" and get addicted to the sex and to that particular person.
Due to the innate inequality of the situation, NO adult needs or has the right to have sex with a kid.
Kids don't need this; and it seems to me destructive because it interferes with the process of their learning naturally, in due course as they grow up, what is right and wrong generally, and for each one of them in particular.
Thus, imo, it's just always wrong for kids to be involved in sex with others.
Last thought: in the distant past (and not so distant past in poor, impoverished countries), children frequently married other children. Once married you'd assume they'd engage in sex with each other. When/where humans only live(d) some 30 years, teenage sex is/was a pre-requisite for the continuation of the species. Under those specific conditions it had to have been a necessary and thus a good thing. (Given that there's pretty much no societies nowadays that have such a short life-span; this type of situation no longer applies).