A child deserves an environment that contains the whole of humanity to form the core of themselves. An environment of man entire in a bond of giving without self interest. Same sex couples can only offer the half of what makes man whole. What are joined when two of the same are together? There is no marriage unless ‘two’ become one. Two of the same are still half of one when man is considered as a whole.

Cannot improve on this. This is exactly my position as well, as both Benadam and I have stated many times on CAF.
To the OP: I would stick with this as your premise, and not drift off into the category of sin & morals. Your opponent will merely say that nonbelievers are not bound by Catholic belief, and that state laws are secular matters, not religious concerns. I wouldn’t even go into ‘natural law,’ as that is a philosophical premise that is shared by many outside Catholicism, but rejected by many as well. Although I personally feel comfortable with the extension of natural law as a basis for civil law, technically speaking the courts have not upheld that natural law is the foundation for American civil law.
I would, however, stick with
nature. (There’s a difference.) I would stay within biology and social structures. I can promise you that many, many adovcates for legitimizing same-sex unions would be horrified at the prospect of manipulating biological structures in other animal kingdoms. It would be considered unnatural, invasive, anti-environmental,
immoral (unethical), and even dangerous. If she begins to go on and on about the supposed “naturalness” of homosexuality in nature, I would point on that such occurrences are limited & specific, in response to environmental pressures of the moment - such as animals of one gender in captivity (which also mirrors human prison behavior, by the way), such as animals using same-sex activity as a limited & temporary means to an end. (Always that end is preserving the heterosexual dominance of the group.)
I would point out that no civilization in history, regardless of how much homosexuality was practiced (both in and out of heterosexual marriage), has ever institutionalized and legitimized homosexual marriage. And for good reason. It has never been considered as something promoting orderly social structures. It has no value in that regard. From a biological point of view, the purpose of marriage is the continuation of the species within a structured environment. (Other animal species very much rely on structure, too.) From an evolutionary point of view, homosexual “marriage” does not further the continuation of the species. In order to continue the species within a homosexual union, one must go outside of the bounds of one’s union, seeking artificial sources (anonymous sperm banks) and/or using heterosexual unions for that purpose: that becomes an issue of the manipulation of other human beings for private purposes. Neither “solution” is intrinsic to that “marital” union, so in itself that union does not promote the continuation of the species.
(I know opponents here are going to pick up on the fact that straight couples go outside of their marital unions in their reproductive choices, but I’m not really discussing here whether such choices should be available to either gays or straights; I, and the Catholic Church, believe they should not be available to gays or straights. I’m discussing whether intrinsically, gay partnerships further the continuation of the species, which they do not.)
The central argument is the one Benadam stated, and this is the most essential thing, universally, across all cultures, all religions (and lack thereof), and all eras: the irreplaceable otherness of male vs female as essential to the full development of the human psyche, and the family unit as the most important location for the development of that psyche. Husband and wife are not interchangeable, mother and father are not interchangeable, male and female are separate identities which are also absolute archetypes.