1) Like the rest of us, they'll probably come to regret it.
2) It'll wound the poor, sensitive egos of religious people.
3) Mumble mumble definition of "marriage". Note that there are some restrictions on who can get married, either by definition or some other reason. For example, close relatives aren't allowed to get married, "fake" marriages for reasons such as citizenship might be rejected, cannot marry more than one person at the same time, cannot marry self, animal, object, or various other things. Why change the law to allow same sex marriage, but not any of these? (social inertia requires a good reason for change rather than a reason for not changing)
4) Mumble mumble "family unit". In much of the developed world, native population is dropping (made up for by immigration, but there's some value in a stable native population). Statistically, marriage encourages childbearing (also, childbearing encourages marriage). Heterosexual couples are more likely to have children (and the only ones who can have children accidentally). Note that allowing couples who can't or don't want to have children to get married isn't a complete counterargument to this, because forbidding such might indirectly mess with other marriages (eg it would be awkward during dating to ask if one is fertile and willing to have children, people might change their minds, and an age limit might encourage divorce just before menopause to find a different partner and old geezers competing with everyone else for younger women)
Conversely, homosexual couples tend to only have children intentionally (and much less often), and are more likely to adopt children. Overall, this may be a more valuable service to society, but it might not be. Also, odds are homosexual couples in the future will have or adopt more children as acceptance increases, and also due to the imminent technological advances that will allow them to have their own genetic children.
5) Out of wedlock children are often a huge social problem, and tend to result from heterosexual sex. Therefore, there's extra value in encouraging heterosexuals specifically to get married.
6) For #4 and #5 above, note that marriage isn't free for the government, due to the cost of government meddling where it probably shouldn't but does. Thus, even if homosexual marriage is more valuable than not, it is still a question of relative worth compared to heterosexual marriage.
In case you're wondering, I took issue with your "zero" reasons. Even if the preponderance of reasons are for one side, it doesn't invalidate the reasons for the opposite side. Overall, I think that
A) The government probably shouldn't be meddling with marriage in the first place, at least not to the extent it is with the various tax breaks and rights.
B) Homosexuals should be not just allowed but encouraged to marry, for psychological and health reasons. (The psychological reasons being primarily that the aforementioned religious people who's egos are threatened by other people being happy, tend to wage psychological warfare on said people).