So: Is there any argument from a secular point of view that can defend our Catholic position in a country where the separation of church and state is a must?
Let me say first off that you are on dangerous ground.
Without the Church, you will be hard pressed to find any firm mooring. The seeds amid the sands and all that.
But, since you asked: Your answer depends on what tack your opponents choose to take. One theory de jour is that homosexuality is inborn, or at least virtually so. This, they say, makes it unjust to restrict a union over people who have no real “choice” than to be other than what they are.
Granting this, it simply does not follow that being inately different entitles anyone to be outside government regulation. Quite the contrary, in fact. If the majority feel that homosexuality is not optimum, and if homosexuals cannot help themselves, then the democratically elected government can enforce the will of the majority on the non-compliant. The fact that the non-compliant cannot exercise free fully due to inate differences makes government action more necessary, not less. We do not exempt child molesters from government regulation simply because they are predisposed to such conduct.
Your opponents will probably respond by throwing up individual rights, or the notion that the conduct is not harmful. These roadblocks are easily navigated.
First, the question has little to do with individual rights. It has to do with majority rights, which ultimately trump individual rights in a democratic system. True, it a balance, but in the end, the majority must prevail or democracy is a lie. That is why the largest voting block carries the election. The majority has the right to structure society according to its will, if the constitutional forms are observed. If you achieve this (and each state that has put it to the vote has restricted homosexual marriage) the democratic question is answered. As far as harm, anything that restricts the exercise of self determination via the democratic process is harmful, because it restricts the liberty of the people.
The only remaining question is whether there is some human rights issue to cloud the matter. If homosexuals are inately different, then any human rights specific to them are analogous to the rights of the disabled. They are entitled to some accomodation, but nothing further. Additionally, they should be rehabilitated insofar as possible. Someone who is blind, or lame, has the right to accomodation, but does not enjoy the “right” to fly a government regulated aircraft, for example. Marriage, as a state created and regulated institution, can be reserved for those fit to exercise its purposes. If the majority define it as a semi-permanent union of a man and women, then homosexuals have no right to redefine it or to enjoy its privileges.