The few secular arguments against SSM that Morse and Gagnon offered either had no direct connection with LGBT people (Morse), or sought to pathologize homosexuality (Gagnon).
Morse spent the majority of her presentation marshaling evidence that marriage as an institution has been eroding over the last 50 or so years—indeed Pew Research shows that the rate of marriage has declined from 72% in 1960 to 51% in 2010. The claim isn't new or controversial in itself, but she links it to no-fault divorce laws, rather than to poverty, limited educational opportunities, or inadequate access to birth control. How SSM becomes another stress upon this country's weakened marital institution isn't clear and Morse makes no case for any causal connection. Doing so would require original research and lots of other hard work, and that would detract from time spent evangelizing. She simply asserts that SSM is different from so-called traditional marriage and that therefore its presence is detrimental to traditional marriage.
Why should we be concerned about marriage's decline? Oh, because married heterosexual couples provide the best environment to rear children, and so the decline of marriage is bad for children, puppies, unicorns, and everything else good and wholesome. To agree with Morse we have to accept a train of claims that she never makes explicit but assumes her audience shares or at least hears despite its audible absence (like a dogwhistle). Let's ignore the fact that many LGBT couples have children, and similarly gloss over the fact that children have been born outside of so-called marriage for as along as humans have walked the Earth's surface, and that the nuclear family is a recent invention. As a member of a large Italian family whose relatives often lived in the same zipcode, if not the same street, I can assure you that many people prefer more communal family arrangements to the particularized, ideal family of mother, father, and dependent children floating alone in their own universe (usually called a house).
Perhaps the decline of marriage and its effect on children is one worth studying, but I don't see how it directly relates to LGBT couples unless we resort to the Thomist line that male and female were meant, designed, intended by God to have sex and procreate. The refusal to pursue that argument and its implicit bigotry is precisely why David Blankenhorn reluctantly supported SSM in the interest of promoting marriage as a secular institution.
Blazing a trail Morse perhaps feared to tread (and thereby alienate the non-believers), Gagnon performed some unnecessarily long and irrelevant (to the debate) hermeneutical readings of select passages from the Torah and Gospels to prove, as it were, that homosexuality is a sin—he even used Powerpoint slides! Missing from the analysis, as usual, were other grave transgressions like mixing meat and diary, heterosexual fornication, masturbation, and other sins his audience likely indulges in despite being good Christians.
He glazed the theological arguments with cherry picked social science research to make them more palatable to a non-Christian audience, as though to say "See! Modern science proves what the Bible tells us!" By claiming that gay men harm themselves by having gay sex (increased STIs, etc.), Gagnon repeats epidemiological studies from the 1990s that gay men have a significantly shorter life expectancy than straight men but doesn't tell his readers (or listeners) how old the studies are or whether any research in the intervening twenty years has superseded the initial findings. Robert Hogg's 1997 article on HIV infection and mortality rates in Vancouver from 1987 to 1992 is a favorite among homophobes looking for any modicum of scientific evidence to legitimate their prejudices, and Gagnon cites it (among many others) in a self-published essay on his personal website, which he dates 2004.
To conclude from Hogg's then 7-year old article that gay men have shorter lifespans means ignoring the fact that article was about the difficulties and benefits of calculating mortality rates using other demographic information (i.e., sexual orientation), and offered no clinical advice on the lifespan of individual gay men. Gagnon would also have needed to ignore another of Hogg's studies published the same year that showed HIV-infected patients on antiretroviral medication were living up to three times longer than those not on the medication. He would also have had to ignore the follow-up letter that Hogg and the other authors sent to the journal's editor in which they wrote that "If estimates of an individual gay and bisexual man's risk of death is truly needed for legal or other purposes, then people making these estimates should use the same actuarial tables that are used for all other males in that population." They felt compelled to write that response after learning that the Family Research Council and other hate groups were using their research to argue against gay and lesbian persons' civil liberties: "Overall, we do not condone the use of our research in a manner that restricts the political or human rights of gay and bisexual men or any other group."
Besides, the answer is not curtailing gay rights or discriminating against LGBT people but providing proper sexual safety education about STI risks, condom usage, etc. We do not yet live in a country where it is acceptable or in accord with the principles of the Bill of Rights to discriminate against people simply because they are statistically more likely to suffer from viral or bacterial infections. If we followed Gagnon's logic, we would be justified in discriminating against the poor because they suffer higher rates of various diseases because they can only afford to eat less healthy food.