In answering this, I'll assuming you mean religious morals when you say biblical morals, given that folks don't always agree in their interpretations of biblical law.
There are three approaches to answer this question.
We assume the secular legislation can be and is distinct from religious law, but that the secular world agrees to adhere to at least one common "secular moral": The survival of society is good.
On this path, we need to make one trivial logical leap about secular legislation. Namely, we need to assume that the purpose of any law is to protect each member of a society from a destructive behavior which can threaten the survival of the whole society. I think it's important to note here that we can only justifiably legislate for or against behaviors based on the immediate, small-scale impact and then treating each individual involved as a representative of the whole society. It is generally impossible to examine a single action and accurately know whether it, in itself, will be the downfall of the society.
For instance: If only one person kills another person in a society of 1 million persons, this behavior does not immediately threaten the whole society. However, with both the killer and the victim as representatives of the same society, the behavior of murder is seen to threaten society. That is, if the behavior were performed either en mass or as an act by and against the society, the society is effectively committing suicide. Hence, killing should not be tolerated beween members of the society, should it wish to survive.
But, we could not reasonably claim that killing is bad for society because a single act of killing will lead to a chain reaction that eventually causes some society-ending calamity.
So, I should also clarify: Only those behaviors which can be "calculated" as threats to society, are shown to have large or growing impact, and which can be reasonably managed are "worthwhile" to actually legislate (probably). We would probably not in sound judgement, for instance, legislate against clipping one's toe-nails "too close" -- even though, if everyone did this, we might see outbreaks of infection that could threaten the society. And while this action certainly could amount to society killing itself with a rusty set of clippers, we have no indication that this is a tendency people actually have, the threat is very low, most of the time it's likely unintentional, and finally, it's nearly impossible to enforce.
According to this line of reasoning, each religious moral must be judged on its own as to whether it fits the purpose of "secular legislation." In doing so, we must, if we are honest, confess that we are finite humans with differing and half-baked opinions. As such, all beliefs regarding the societal impact of a behavior are justified in being nominated for legislation.
As Christians, it's no leap at all to suggest that our religious morals not only overlap in the goals of survival, but that our morals are very good for the secular mission. And since we're all just folks with differing, half-baked opinions, the strict secularist, is he's honest about his own limitations, will have to serious qualms allowing "divinely inspired" behavioral insights into the legal forum.
We don't need to prove, for instance, in any scientific study or grand philosophical discourse that adultery is bad for society. It should be sufficient to state that "we have a very good sense" that it's destructive, whether by our own intuition or by divine inspiration. This doesn't mean the society at large must agree with us on any point -- only that we should feel "justified" in bringing important morals to the table.
We assume that, since secular legislation must be founded on at least one commonly shared moral value (the survival of society), which in itself is a religious value, all legislation is inherently religious. This is a much more philosophically sound approach, in my opinion (though it obviously won't appeal to the strict secularist). But, the root of this approach is this:
There is nothing in nature, nor in the scientific method, to insist that existing is better than not existing.
So, when a society agrees that continuing to exist as a society is good, they're already making a religious statement. That said ... well, we don't need to go into more detail. The society is already religious. Religious values, at that point, are rightfully the driver in all legislation.
Good luck convincing any strongly secular person of that though :) Hence, I presented a First approach that doesn't make this "assumption" ...
Perhaps more to the heart of what I think you're asking, God sows His seed everywhere, even where the soil cannot welcome the seed. Regardless of what a religious person feels the nature of secular legislation ought to be, it ought to be the devout religious person's inclination to save everyone. Hence, the devout religious person's motivation should be to bring their religious morals into the public forum, via legislation if needs be. (Legislation shapes the conscience of the people!)