Christians don't believe there's objective morality.

[genkaus]

I don't understand what you mean? The is-ought distinction is a distinction. Between facts and values. Science tells us what is, not what ought to be, and it can't do so.

Study up on it. The is-ought problem, as devised by Hume, talks about the gap between "what is" and "what we ought to do" on basis of it. There have been several response to it including the use of goals to bridge the gap or using institutional facts.

The idea of grounding it in biology is just the least absurd option.

Really? I would've thought that psychology would be the best option.