Doesn't that increase the governments role in civil society. Wouldn't it be better to get government out of heterosexual marriage? This would also appeal to religious people because we can say to them that the government will not be using their tax dollars recognizing gay marriage qua gay marriage. It would still, of course, be recognizing the underlying civil contract.
Ideally, the government will go away, completely. Not living in that ideal world, yet, we have to deal with what is.
The government's power is to prohibit. Currently, in most places, they prohibit gay marriage (either directly, or by heavily penalizing gays who get married in various ways). Passing gay marriage into law, removes their ability to prohibit it, thereby
reducing their power.
The only way we will ever get the government out of marriage, is to end the special privileges. As long as heterosexuals have a special privilege that others can't have, many of them will fight to defend the system that gives them their privilege. Once that privilege is ended, they no longer support the government-controlled-marriage system as strongly. This isn't just theory; we've been doing it, here in NH, and as civil unions and then gay marriage were passed, support for getting the government out of marriage has increased substantially. Many of those who previously supported that system only to defend their own privileges, now support it less or not at all, because those special privileges are gone.
Privilege ("private law" - special legal "benefits" given to some group, but not others) needs to be ended, before many of those who are at the receiving end of that privilege will even consider opposing the system. That's why the government is so keen to get so many on some sort of welfare, etc. - he will then defend them in order to maintain the privilege. Other than bloody revolution, which any sane person would prefer to see as small as possible, or avoid completely, ending privilege is the only way to shrink the government's power.
Joe