You've probably had the experience of seeing a film and enjoying it, but then on a subsequent viewing rather disliking it (the film "doesn't hold up"). Or the reverse -- hating the film on the first
viewing but then enjoying it in subsequent viewing (you finally "get it").
In my last post on ethical relativism, I argued that species categories are
vague, and that ethical universalism must therefore be an empirical claim about
the contingent constitution of members of Homo sapiens.
Consider this description of an imaginary species I'll call Homo atrox:
Homo atrox is in most respects similar to Homo sapiens,
but with a far more pronounced disposition toward cruelty. In fact, studies
show that atroxians who engage in wanton acts of exquisite cruelty lead
substantially longer, happier, more productive lives than those who
try to abstain from cruelty. 
Conventional ethical relativism appeals to some set of social norms,
or practices, or personal convictions as truth-makers or grounds for
moral justification. On this model, for any moral precept p, the
statement 'p is true' means
something like 'I (and/or those of my social group) accept p' -- where
such acceptance in the
assertor's moral theory (perhaps linked with some anthropological and
psychological theses) constitutes the relevant "truth" or
justification of p.