Right or wrong...good or bad.. who decides

just this other day had a good n thoughtful debate with some friends of mine..it actually started off on valentines' day when a friend jus casually mentioned how he thought going around/dating without any strings attached (..or to be attached) was wrong... morally n ethically.. his point was you propose to a person of the opposite sex only with the intention of marriage..meaning if u see someone..get attracted to tht person it is because u see him/her as a potential life partner.well my point being exactly the opposite meaning marriage is (n should not) be the only criteria for proposing to someone..the point i made was tht jus a casual relationship is harmless n more importantly NOT wrong.. to which he said it ws against our culture.. now we debated a lot on this culture issue with case studies n real life scenarios.. bt with due course of time we were led to another altogether different topic.. who decides whats right/wrong?or something is good/bad?? ohk i know ppl would say ur ethics/principles/moral values,etc.. bt dont give me tht hogwash.. for a moment think of how u can judge a certain act/belief/ritual as being right or wrong in absolute terms.. i mean for the debate above my friend says a casual relationship is wrong.. n i say its absolutely fine n normal.. but like who decides whether it is REALLY right/wrong, good/bad.. its a really controversial topic i know.. but needs lot of maturity to think on it because when u form an opinion on something its based on ur personal bias/experience/culture,etc (meaning it is a relative thing).. BUT does tht mean tht ur opinion is right.. there will always be some ppl who share ur view who feel ur opinion is right..BUT thr will also be ppl who think otherwise.. now who decides who is right among the 2 in absolute terms..
take the case of alcohol consumption, non-vegetarianism, robbery.. all these r issues whr thr is a general consensus tht they r wrong.. but i want you to think WHY r they wrong??keep ur religion/bias/principles aside n think of the answer..
i have tried to burst my brains thinking about this but have not come to a conclusion..does tht mean all tht is in this world is relative?? thts the big question..
plz try n share ur thots/opinions with me would really appreciate that...