Because you're putting forth a premise that one's genes make a person behave in a certain way. Instead of the devil compelling one to do something, it's one's genes that do so. The net effect of all that is negate absolute truth and personal responsibility, which an objective of leftists and secular humanists.
So, if genetics can cause one to have deviant sexual desires, why couldn't they give on a desire to violent and murder?
In fact, if you think am I just making up wild analogies, you're wrong because "scientists" have been trying to find a murder gene as well:
http://www.time.com/time/magazine/article/0,9171,1004083,00.html
I guess we’ll never really know then, it’ll just be a matter of what one believes.