There are two reasons animal behavior is studied. The first is to gain understanding of the natural world, sometimes to control it. The second is to answer ethical questions about human behavior. The first is useful, the second misguided.
The reasoning might be that, because some animals engage in same-sex sexual activity, it is natural for humans to do so to, therefore not morally wrong. However, there are different, equally valid inferences which can be drawn from the same data.
One is that "homosexuality" in animals - and indeed sexuality in general - is more about social bonding than reproduction, and therefore sexuality in humans is similar, and therefore social restrictions against homosexuality, polygamy, and "Sluttishness" are counterproductive.
A third possible interpretation is that animals show a moral degeneracy, contracted from contact with immoral humans, and therefore humans should try to set them a better example. A fourth is that homosexuality is part of primative animal sexuality, and therefore humans who do it are unevolved brutes.
All these lines of reasoning are nonsensical because they try to derive 'what ought' from 'what is' - they try to base ethics in empiricism. Indeed, they treat each fact as having an indissoluable ethical value - that animals are 'less evolved', that human society is 'corrupt' or 'ungodly', or that 'alturism is desirable'.
Facts do not contain values. Values are not a type of fact. Facts can be observed and interpreted, values can only be ascribed.
Another problem is the selectiveness of the reasoning. Someone may argue that, because some anthropoids practice something analogous to polygamy, the notion of monogamous marriage is untennable. The problem is that some anthropoids practice infanticide, but parallel reasoning is not used here.
It is entirely possible to reach a correct (or "just") moral conclusion be reasoning invalidly from correct empirical statements. Conservative attitudes towards homosexuality and polygamy are indeed incohearant, insupportable, and do great harm. My point is that using evidence from animal behavior to attack these positions must fail.
It fails because (a) it is illogical and (b) diametrically opposed conclusions can be drawn using the same illogical reasoning from the same evidence.