I am not an expert on every verse of the Bible, but I don't recall there being a passage that forbids a believer from fucking roadkill. And even if there is, I'm confident that he doesn't know about it, either. So, I suspect that the actual reason that guy doesn't fuck roadkill is nearly the same reason that I don't fuck roadkill.
I don't fuck roadkill because I don't want to. He, on the other hand, also doesn't want to, but he goes the extra step to say that because he doesn't want to, and it's sexual, then actually it's God that doesn't want him to do it.
In my experience, Christians do this frequently. They have some feeling, and then assume that God must want them to feel that way. I can only say that the thought itself must be blasphemous. And when you look at it in that light, it's actually pretty blasphemous if that Christian doesn't stop to fuck all of the roadkill he comes across. Who is he to guess the mind of God? Even if he doesn't want to, he's got to go pump his meat into that dead deer.