If there is no objective morality, how could we determine morals. What is moral today may not be considered moral tomorrow. In future, even pederasty, pedophilia, bestiality may be considered normal. Greek cultures have accepted pederasty, hedonism in the past. What do you think.
We could determine a relatively objective ethics (morality is much more general) by appeal to scientific materialist criteria. I set out many of these both in a book published in 2000 (Ch. 6 of 'The Atheist's Handbook To Modern Materialism') as well as an essay appearing in The Barbados Advocate Newspaper in 1991.
As I pointed out in both, the true scientific Materialist, by definition, respects his fellow men and reveres all life, since he recognizes (through his philosophy) that they share a planet that may be unique in the cosmos. Thus, the true Materialist treasures and conserves the Earth's finite store of resources, since he comprehends that Earth also has one life to live - and there is no more after the existing resources are consumed.
Embodied within the preceding there abides a practical ethics, forged out of the Materialist's reason and his priorities. This has one overriding aim: to cherish the Earth and all life upon it. Consequently, the true Materialist disdains all forms of violence, since ultimately these are inimical to the community and to species' survival. Since violent acts undermine a community's cohesion and threaten its very existence, the true Materialist must regard them as irrational. The Materialist is compelled to co-operate with his fellows and promote a common good, not out of fear for the wrath of a deity, but to held insure a thriving, harmonious community with high survival value.
Given such acts as pederasty, and bestiality would be clearly inimical to the ethical objective described, these acts would be definitely unacceptable and incur sanction.
BUT as ethicist William Provine notes in his article 'Evolution and the Foundation of Ethics' , young people should be encouraged to think rationally and critically concerning ethics, not out of fear of some divine force's wrath, but to protect their own long-term self-interest.
Any persistent observer of human social interaction will note that the vast majority of people are law-abiding and decent folk who naturally practice a common-sense, utilitarian ethics similar to what has been described. For proof, one need only look as far as the upstanding Atheist or agnostic who inhabits every community and who - though he disdains a deity, nevertheless treats his fellows with compassion and respect. No supernatural law or commandment ordains this behavior. Instead it is the conscious and deliberate recognition that the promotion of the welfare of others is directly linked to the one's own welfare.
Unfortunately, what the religionists have done is to take the natural code of ethics most people follow and embellish it with a blizzard of superstitious precepts and injunctions. These are superstitious since, inevitably, they are linked to the supposed dictates of a supernatural "being" who will not hesitate to "punish" those who disobey "him".
Christians, for their part, profess a ‘God of love’, but never hesitate to invoke fear (of eternal torment) to have their ethics adhered to. Logically, this suggests that the ethics is insupportable without the additional imposition of some type of "divine" retribution. A punitive ethics, then, is at the very core of the Christian religion, as it is many others. Sir Bertrand Russell, in his book Why I Am Not A Christian, precisely identifies ‘religion’s source of terror’ to account for the hold it has on so many. He notes how fear has been ‘dignified’ by use of this source: the demented hell concept to the point people no longer think it disgraceful
I think you can also see from the above that situational ethics as you describe, may not also meet the criteria of Materialist ethics. For example, pederasty that undermines the integrity(and survival) of a community today will still undermine it 100 years from now
 MBL Science, Vol. 3, No. 1, p. 25, 1988.
 Russel, R.: Why I Am Not A Christian, Touchstone Books, p. 54, 1957.
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Thanks for your reply. If it can be scientifically proved that some animals derive pleasure by having sex with humans, how could bestiality can be wrong. Some people in western countries are already pushing for its legislation. If hedonism, wife swapping is accepted in society, how it can be argued that such life style is immoral.
Apart from the fact the hypothetical is totally absurd "if it can be proved" etc. it would still not meet the criteria I provided for an objective ethics. That is, it would destroy human community and commonality as surely as pederasty. Concocting weirdo scenarios, in other words, doesn't get us any closer to discussing ethics in a rational or consistent way. In like manner, wife swapping would never work nor could it be ethical given the same criteria. But to in any way analogize wife swapping exactly to bestiality is absurd.
Again, I am not venturing into "morals" where moralism is also widespread. For the most part, moralism isn’t founded on species survival but on an unsubstantiated and subjective belief in a uniform human sensibility to external stimuli. In this way it becomes the conscious vehicle of behavior modification, used by any group with political or economic power.
As an example one can cite masturbation. Objections to this act are almost uniformly based either on moralism or an outdated understanding of Catholics’ natural law, which is neither natural nor any kind of recognizable law. The putative basis for it more appears to be the presumption of some kind of natural moral order which is theoretically known to humans and so their knowledge will lead them to avoid infractions. For the scientific Materialist this is nonsense, especially given that at one time slavery was embraced as acceptable under natural law.
Masturbation is a controversial but important example of a theological natural law proscription, with near universal applicability. It’s also very useful in an era of spreading HIV-infection, AIDS and the recent campaigns for teenagers to embrace virginity (abstinence) and ‘just say no’. This is where masturbation enters as a sensible option to teen sexual intercourse. Note that for a Materialist, abstinence is only applicable to intercourse. Absolute abstinence (i.e. from all sex) is regarded as not only impossibly utopian, but quite probably harmful.
In terms of the criteria given earlier, there is no demonstrable harm arising from masturbation, nor does it harm anyone else by definition. Moreover, once done in private (as is normally the case) it does not violate any civil laws. Hence, the Materialist regards it not only an innocuous act, but also a highly desirable alternative to casual intercourse with its inherent risk of HIV-infection, and unwanted pregnancies.
In point of princip0le. millions of truly abhorrent acts might be considered but that does not make them unethical or immoral though they might be repugnant from a moralistic stance. For example, Dacher Keltner in his book, 'Born to Be Good' (one section)was interested in exploring the arousal of what he called "gut morality" in an Ethics class. He surmised most people simply reacted at "gut" level before their neocortex could process actual moral transgression and distinguish it from idiosyncrasy - even foul idiosyncrasy. To that end, he asked students to morally rate a scenario in which a man goes to purchase a chicken from the grocery each week, then has sexual relations with it.
After most of the class reacting emotionally and screaming "Pervert!" and condemning the act as most foul (not fowl), Keltner led them through the act again and more slowly, engaging their higher brain centers instead of the limbic ones. By the end of the period they backed off, dismissed the guy's penchant for store chickens as a foible and agreed no law or moral edict was violated so long as the practice was confined to his own domicile, preferably with the curtains drawn. Oh, ...and he should not invite any guests for chicken dinners!
My point is that it's important to be able to distinguish - when asking questions - moralism, from morality as well as from ethics, and also not to interject hypotheticals that are not consonant with any of these nor ever can be. In the latter instance, no matter what "scientific proof" is ever offered you will never ever see bestiality even accepted as a merely repugnant example of moralism, any more than it would constitute an example of acceptable ethics.