Why? Essentially, while it is a clear vote against gay marriage and would put the question before voters in an off-year election when the issue is still lukewarm, they don't want civil unions either. So instead they are pushing for a third, far less popular option that bans both marriage and unions.
What seems pretty clear to me is that they are risking diluting the vote for something that has a slim chance of passing in favor of something that won't pass the legislature or be approved by the people. They have given people a free dodge-the-conservative-shitlist card, now they can vote against the "no marriage/yes unions amendment" and vote for the "nothing amendment" while the votes opposing either amendment will outnumber votes in favor. They even risk making the rejection of the "no marriage/yes unions" appear to be even stronger than it actually is among legislators. They are defeating their own cause with their stubborn, extremist myopia.
However, pro-gay marriage activists in other states could learn a lesson from these conservatives. Their all-or-nothing strategy reminds me of same-sex marriage supporters who won't ever consider settling for civil unions in the short term in states that are strongly Democratic but people are still turned of by the term "gay marriage." Instead of going for the smaller, incremental victory they are holding out for the unattainable one. I worry that the groups who push for "full marriage rights only" only help constitutional bans get enacted which pushes away any chances of any familial rights for gay couples in the near future.
Neither side seems to understand the importance of disarming your opponent and fighting smart. Sometimes you fight for what you can get when you know you can't get what you want. Something is better than nothing and sometimes pragmatism should trump idealism. Ignoring the fact that politics is a game of strategy is the first step to losing the game. You don't win anything by losing everything.
At least today it looks like gay marriage in Massachusetts is here to stay-- thanks to the myopia of some Massachusetts conservatives.