Had theistic evolution been true (in the sense of humans being descended from sub-human primate mammals, but all supervised somehow by God), this would imply that God approved of fornication, even though the Law later given through Moses forbids adultery.
You are assuming that the behavior of animals today hasn't changed in the millennia since evolution and/or creation occurred. Since that's the whole point of evolution, I'd prefer to disprove it without having to ignore it completely. Fro...m what I can tell, God provides a 50/50 gender split in every species. There is nothing preventing monogamous relationships in any species I'm aware of. That is, no species on Earth needs an adulterous/fornicative relationship to pro-create. If you believe that the object of sex is to improve chances of survival by mixing up DNA, then why hasn't a more robust method of procreation evolved? A 4 way mixing would lead to quicker evolution. Especially in the "lower" animals (fishes/insects/amphibians) where 'sex' occurs outside the body, you'd think that allowing more than 1 pairing of DNA would have evolved, since by the rules of evolution it should have been the 'stronger'. Where is it? The theory of evolution loses steam when you start looking at what it should have produced instead of trying to piece together God's quilt in series instead of in parallel.
As to the point of saying that because some animals fornicate today means that God didn't use evolution eons ago... If the deceiver was able to get to man, isn't it possible he also corrupted other creations in the earth? Is there any proof of fornication prior to the creation of man in the written or natural histories that I've missed? I'm not saying this is the case, just that it's a possibility.
@Michael: Very helpful contribution to the discussion. What you are saying is in effect that non-monogamous sexual relationships in mammals may be a result of the Fall, and thereby part of the "groaning of all creation" which Paul refers t...o in Romans 8. This is an interesting idea that I had not thought of before. Yet to state it in these terms does also militate against Evolutionism as a species of Philosophical Naturalism. The judgment following man's disobedience had far wider effects than most naturalists assume as a working hypothesis, even Christians who study the natural sciences.
There's a difference between suggestion and evidence. The points about fornication in other species being either evolved or fallen behavior are suggestion, not fact. Without time travel, there will be no evidence to prove or disprove it. It's a matter of faith either way.
I do see universal evidence today which supports an original design of 1 male:1 female in all the animals on earth. "Natural selection" does not allow for such wasteful creations like 50% male births in walruses if only ~2% (*) of male walruses ever procreate. If Darwin had it right, then the birth rates of males in walruses should have dropped eons ago. The fact that males are born 50% of the time seems to be evidence against natural selection and evidence for an intelligent design that originally planned a 1:1 relationship for all creation, even where it wasn't necessary for the survival of the species. When you look at all the facts, the leap of faith that says the social behavior of walruses doesn't match their design is much smaller than the leap of faith that says there is some "natural selection" reason for the 48% of the population which has nothing to do with survival. The Law of Moses is confirming evidence that there is a reason for the unscientific birth rates in mammals.
(*) My 2% statement here is purely a guess based on public television shows. It should be verified if it's going to be repeated.
When you start talking about Man being an 'evolved' creature (with or without design involved) vs. built from clay on day 6, You get into the realm where again, without a time machine, you'll never be able to convince everyone. It is important to point out the leaps of faith that 'evolution' requires are much much larger than the leaps of faith that 'creationism' requires.
To me: I find it entirely possible that God 'created' all the history that was scientifically required for man to exist in the blink of his eye. As He knew the hairs on your head before he made you, God lives outside time, and trying to understand 'creation' from inside time is going to be incomplete in any form we can grasp it. That is, in one (second, minute, hour, day, eon, other time period), He made the world which had evolved for eons up to the point of dinosaurs living up to today. And then, in another (blink of his eye,) 65 million years ago there was a huge asteroid which wiped out the dinosaurs, and all the events required to make man in the likeness of God. None of it happened, it was just what was required for man to exist. God made it already completed, already done. To better explain the difference between the earth being the result of the Big Bang or Creation: Pretend time machines existed and you set the destination for 67 million BC: If the big bang theory is correct you'll open your door and see T-Rex. If creation is correct, you're going to run into a huge brick wall about 6500 BC and discover all the stuff before that is an image, and not history. However, from here (6500 years or so into the creation), looking back you will never be able to see the wall. To all of us inside the event, created history is the same as transpired history, and arguing about which is REALLY correct is just over thinking it. Does it really matter if dinosaurs actually roamed the earth or the earth was made with the history of dinosaurs in it?