Frankly, I doubt arguments from religious doctrine or natural law will gain much traction with a secular person.
A neo-Darwinian/psychologically based argument would be that:
A) Obviously, two men or two women who are exclusively homosexual cannot reproduce. Even the unusual set of circumstances of a gay male “crossing the line” with a lesbian for the express purpose of fathering a child is a heterosexual act, and arguably, the use of artificial insemination is technically heterosexual, in that an orgasm provided semen which was inserted into the female for the purpose of reproduction, an act which is closer to heterosexuality than homosexuality, as both a male and a female must be involved.
B) Traits that reduce one’s ability to transmit one’s genes - such as infirmity, feeblemindedness, physical unattractiveness, infertility - are generally held by all human societies to be undesired traits. They are not traits one seeks out in a potential mate, they are not traits celebrated by society, and they are not traits one desires for oneself.
C) By accepting an identification of oneself as “gay,” one has accepted (consciously or unconsciously) that one’s genotype - all the physical and behavioral traits that make us unique - is unworthy of being transmitted to another generation. This devalues the self, and at some level, leads to the loss of self-esteem and sense of personal dignity and worth that may be a contributing factor to many of the societal ills associated with homosexuality - substance abuse, extreme promiscuity, risky sexual practices, higher suicide rates. While the gay rights movement insist that the root cause of these recognized problems is lowered self-esteem due to bullying and homophobia, this model suggests that the problems themselves may be insolvable even if society were to eliminate such actions.
To the extent that a person who self-identifies as gay is able to have a child (whether through adoption or through a heterosexual act), he or she may be able to assuage some of the feelings of lack of self-worth, but the fundamental psychological identification of oneself as “gay” is an implicit identification that one’s unique self is unworthy of transmission to another generation, unless one uses a generative heterosexual act (one’s own or another couple’s) to gain esteem.
It could be argued in extremis that technological advances such as cloning may permit two men to reproduce, but in actuality, it is the reproduction of only one person, not a couple (barring gene-splicing technologies that still lie in the realm of science fiction), and is clearly a societal ill due to the reduction in genetic diversity in the species.