I must begin by stating that I am not a proponent of "marriage"; unlike many, I feel that the patriarchy has NOT particularly benefitted women and children historically. Having said that, civil unions are not equal but separate.
There are around 1,000 federal rights granted to married couples, and many important ones cannot be gotten by spending money on an attorney. Even if a partner gives medical power of attorney to their partner, family can demand that there are no visitation rights.
There is no family leave, social security benefits, right to inherit, or right to retain the family home if one partner goes into a nursing home. These are just a few.
As far as the "slippery slope" argument, the ridiculous argument about people marrying their dog, marriage is a contract between consenting people; as far as I know, animals don't have the right to enter into contracts. This argument is a long one as far as I can see.
I have friends, a lesbian couple with three adopted children and two biological. The partner that works from home, as well as her child, stand to loose health insurance if the Michigan ammendment to ban same-sex marriage is adopted. How will this benefit anyone?
Where is the line drawn? Where society draws it. It's moved in the past to include women's rights, civil rights, protection for children from slave-like conditions, interracial marriage, smoking, and more. Some have felt threatened but the line has still moved to the benefit of society.
my 2 cents,Emma