You used the word "animalistic" in the way you'd use "beast-like" or "monstruous". If you're being animalistic, it would rather mean you're using your instincts or intuition rather than strict reason. Animals think that way. And what makes you think animals have bad behavior? They love their babies, just as we do, they attack when challenged (you argue when someone insults you), and defend their possesion/territory (people sueing other people for the same reason).
We do act like animals, just that we found different ways of showing it. Nature has rules, which conditionate survival. You're strong or very clever, then you survive. For us, it's like if you stick to the rules of society (i mean knowing people, having a sense for finding ways to reach your goals and things like these, not laws), you prosper. Look at ants. They're very civilised, yet simple.
People look like beasts when they give up thinking they're in a society that demands rules (which can be good or really bad), and they follow their own ways. When their new ways conflict with existing ones, things get dirty.
You mentioned sex. You call animalistic what scientists called deviations, which are somewhat normal (because they're common). If these deviations are to be accepted, well, it matters what kind of deviation is in discussion (and it's not on this topic), some are acceptable, others are not.
People were always acting naturally, even though they concealed their instincts inside their reason.
And here's something you might not like :P. You asked this question so you'll find a good answer for you, so it won't trouble you, so you won't waste any more energy with it, and energy is very valuable (a fundamental principle for animals).