Are accepted social norms and the law of the land moral sexual conduct according to Buddhism?

The flip side of what Mahakhema said is that society may forbid various behaviours. And I get the impression that if something is forbidden by society then you should consider that before acting, for example:

• If it's a crime in your society to work on whichever the "holy day" of the week is, then maybe it'd be better not
• Perhaps more to the point, if for example it's a crime to marry somebody when they're aged 12 or something, then again you'd better not -- even if you did (theoretically) have their parents' permission, and/or even if there's nothing explicitly forbidding that in Buddhist scripture.

I'm not sure how to prove this by reference to scripture -- the suttas don't say very much about lay ethics. Terms like Kamesu micchacara and Abrahmacariya are used (in the precepts), and sometimes defined in a sutta, but even so different people and different societies may have different opinions about how that's applicable in the modern day -- i.e. there are various fringe topics or edge cases, for example prostitution, and "dating" outside marriage, and so on.

The Vinaya is a lot more detailed, and of course according to the Vinaya all sexual activity is forbidden to monks. But this might be a clue -- page 1069 of Ṭhānissaro Bhikkhu's translation and explanation of The Pāṭimokkha Rules says that there are various reasons why someone might be ineligible to become a Bhikkhu, including being some kind of criminal:

• Someone with an arrest warrant, or is on probation or parole

• An escaped criminal

• A notorious criminal, unless they're well-known for having amended their ways

  Minor criminals who have not been caught and have abandoned their criminal activity are not prohibited under this rule. This prohibition was inspired by the public reaction to Ven. Aṅgulimāla’s ordination

There are other places in the Vinaya which reference civil law.

I'm not trying to imply that "anything tolerated by society is considered to be moral according to Buddhism" -- which might be how you're understanding the quote.

Even the opposite -- i.e., "anything forbidden (or required) by society is considered to be immoral (or moral)" -- isn't necessarily always true, for example laws of the State which require you to kill might be considered immoral.

But avoiding extremes, maybe it's right to (as quoted) at least consider "social norms" and "law of the land" as one of several relevant "angles" when "looking at" sex.

Also the quote doesn't seem to be about "Buddhism" (as in the title) but about the Buddha. It might be presumptuous to claim to understand all of the Buddha's thought-processes, but the doctrine does seem to consider social norms and laws. For example, this answer includes a quote from the Commentary which explains what MN 41 means when it describes someone as "protected" by their parents. That social norm was presumably contemporary then, and might be universal too (i.e. more or or less true in every society, except perhaps slavery).

Buddhist morality has one goal (suggested in the first bullet point of the quote): to set up the conditions in which attainment is possible. For those who embrace the restrictions of the monastic life this goal is simplified, because the purpose of monastic life is to create a social context that supports and maintains the correct conditions.

Lay practitioners face a more complex moral problem, because they choose to live in a world that does not always (or even often) create the right context for attainment, and in many ways actively opposes that goal. The wider world is driven by karma: by fixations and attachments, by fears and rages and desires... One must step carefully in the wider world, or risk getting caught up in its lunacies.

In this sense, paying attention to 'social norms' and the 'law of the land' is useful. Norms and laws represent reactions: responses to things that people in that community fear, hate, love, have pride in, etc. In other words, norms and laws are tanhā that have congealed into public institutions; like all tanhā they are based in ignorance, but like all tanhā direct confrontation can produce strident, forceful opposition. I don't mean to suggest that we should be blindly obedient to all norms and laws — we don't need to surrender to collective ignorance — but transgressing a norm or a law is an act with extensive consequences that should not be taken lightly.

It's common in the wider world to see one attachment raised against another, the 'rebel' fighting against the 'status quo' until one or the other collapses in exhaustion. Lay practitioners risk falling into that 'combat' stance, which isn't consistent with buddhist practice. On one hand we have the attachments of others that show up as these kinds of rules; on the other hand we have our own attachments that lead us to cross these rules. We need to fold those hands together to act in dharma.

Both social norms and laws are the result of a balance between personal freedom and the necessary restrictions imposed on those freedoms to ensure that the social order is maintained.

Or in other words, norms and laws represent some kind of balance within a social group between the desires of each individual, against their most obvious problematic outcomes for society at large.

In a social context where people can have wildly different desires, the satisfaction of which can have good and bad outcomes for other individuals, some limitations must be put in place, or the society will destroy itself. This is not the way the Buddha’s teachings arose.

The Buddha taught how to end personal suffering by learning how to end the craving and attachment for things that we cannot obtain, do not have, or should not desire, each of which cause us to suffer.

Thus, the goal of the Buddha was focused on helping each individual to have a better life, but in a way that also ensured harmony between people, because individuals in conflict suffer as a direct result of conflict. And if a society was based on these same principals, then there would probably not be a difference between how individuals comport themselves, and how society needs them to comport themselves.

But, most people, even some Buddhists, are ignorant of, or fail to properly apply the Buddha’s teaching to their own lives, so a society that truly implements the Buddha’s teaching is presently one of those things that a desire for will cause us to suffer, because our desire will be frustrated.

So no, you truly can’t rely on social norms and laws to be a guideline for what proper sexual conduct is from a Buddhist perspective. But you can be assured that sexual conduct that conforms to Buddha’s teaching will not violate social norms or laws.