I have read, but do not know where (perhaps in Demonic Males, perhaps in the book on Rape by Thornhill and etc) that in Papua it is relatively common for a man to rape a woman he is interested in if he has nothing much to offer. She is pretty much then only able to marry the guy who raped her.

I should perhaps try to find a reference.

But perhaps some evidence that the seriousness of attempted murder is not a constant across cultures.

I see this as evidence of what great shame it is to the family for a daughter to have sex outside of marriage. By marrying the man who raped her the same is lifted from the family. In other cases in Turkey family members have murdered their daughter after being raped.

"If a man happens to meet a virgin who is not pledged to be married and rapes her and they are discovered, he shall pay the girl's father fifty shekels of silver. He must marry the girl, for he has violated her. He can never divorce her as long as he lives." (Deuteronomy 22:28-29 NIV)

So never mind an omnipotent God, it is being 'discovered', i.e. the shame of other mortals knowing, that is the problem.

"If there is a betrothed virgin, and a man meets her in the city and lies with her, then you shall bring them both out to the gate of that city, and you shall stone them to death with stones, the young woman because she did not cry for help though she was in the city, and the man because he violated his neighbour’s wife..." (Deuteronomy 22:23-24 NAB)
That comes a little before the verse quoted above.
Hence, if the attack happened in the countryside, and there was no one to help even though she screamed, then she is not guilty and will not be killed, while the rapist could be. How often would rapists in such cases have been executed? What could be proved? The assumption seems to be that if the rape occurred in a populous area, some civic minded soul would surely intervene at the sound of screams, ergo, there's no such thing as rape in a populous area.
No wonder some people become atheists if they think God wrote this stuff. Actually, it's humans grappling with the dark and shaky side of human nature. God really doesn't come into it. And no wonder brides of the area, down to this day, keep chicken blood on hand for their wedding night.

I've heard of a similar custom in Sicily, and in Kyrgyzstan it's supposed to be one of the commoner ways marriages are contracted (fake abductions are sometimes arranged with the consent of the parents, and perhaps sometimes girls run off voluntarily, but real abduction and rape are common -- the linked article from an advocacy group says 50%.) There's also a custom like that called moetolo in Samoa and the Pacific Islands (an issue in the Margaret Mead controversy).

It's really hard for us to imagine these things from our individualist point of view. In many traditional societies, individuals were not independent actors with their own interests, but part of the kin group. Only the clan leader acted autonomously, for the good of the clan as a corporate group (though he might identify that with his own personal interest). The lives of individuals were decided on in terms of a corporate interest.

On top of that, in many of these societies women are always subordinate to males and are almost always treated paternalistically, and not as agents.

Anyway, the contrast is really with arranged marriage. In the Kyrgyz system (traditional among Mongols) there's a battle between the girl's clan and the groom's clan, with the girl just a stake in the game. (Abductions are carefully organized). It's closer to the war of each (clan) against all (clans) than it is to the controlled world of arranged marriage between clans. It may even give girls an option of running off voluntarily, but (in Turkey, I'm not sure about Kyrgyzstan) her family can still take revenge on the groom's family if they want to. (The woman is ruined in that case anyway, however. She is not a player and doesn't have an interest; the clan is protecting its own reputation for strength).

The shock people feel about these things is a bit ahistorical and uncomprehending (note that it's there in the Bible too, not just in the customs of contemporary foreigners.) At the same time, this is an excellent example of the limits to the possibilities of "cultural diversity".

But perhaps some evidence that the seriousness of rape is not a constant across cultures

Or maybe just an example of people trying to make the best of a bad situation?

It is really bad example. Which would give courage to people like that boy, for doing things like that. But in most of the cases, we have to think solution for our own problem(who want marry that girl?), rather think about in large perspective, which is inevitable too...
Car Roadside Assistance Students

In the old Scots and English ballads, there's often a very blurry distinction between seduction and rape (and prostitution, for that matter). One typical way of resolving the issue:

If he be a married man, hangèd he shall be,
And if he be a single man, he shall marry thee.

I don't think the issue here is whether or not rape is considered as serious an offense in different cultures, but rather the value women have in a particular culture. I doubt you will find one culture where women are highly valued and yet rape is not looked at as a serious offense.

Women are highly valued in rape cultures, but don't have autonomy. They're some man's property.

The difference between Chinese arranged marriage and Kyrgyz rape marriage isn't that the woman is respected. In China fathers control their daughters' marriages, and among the Kyrgyz they often don't. In China girls are still often married off to strangers against their will.

Another (if closely related) element might be that cultural understandings of sex vary across cultures to some extent. Where does this thing called "sex" begin and end?

This became clear to me during a visit to North Africa. We tend to think, much of the time, that it has to do with penetration -- ie., the girl gets to say "no" at any time up to actual penetration, when I guess we assume that it's underway, there's no going back. But up to that point the girl gets to choose to refuse, and if she's overpowered then, well, maybe it might be rape.

It became clear to me that that set of understandings doesn't hold for the North Africans I observed. As far as they were concerned, if eye contact was made, if skin was shown, if a certain amount of lack-of-inhibition was demonstrated by the woman, then sex was already underway and there was no turning back.

As you'd imagine, this led to all kinds of misunderstandings, given the tendency for Euro tourist girls to show up wearing t-shirts and shorts and acting all free and easy.

North African guys felt completely entitled to proceed ahead as though permission had already been granted -- after all, any woman in their culture carrying on like that wasn't just asking for it but was already committed to it. Meanwhile, the visiting Euro girls weren't even aware they'd yet ventured into "possible" territory.

Also helped account for some of the boiling rage you could feel coming from the North African guys. As far as they were concerned, they weren't just being offered tempatations, they were being given promises -- and then denied what they felt they had every reason to expect.