Because our very existence challenges the gender stereotypes that mold so many heterosexual marriages. Since the majority of people are heterosexual, their lives are well served by the current system. Same-gender relationships, no matter where they are on the scale of conventionality, don't fit into the heterosexual model, making them inherently unconventional. Raising the legal status of same-sex relationships to the level of heterosexual relationships represents a radical societal shift. When they are thinking about same-sex couples, it is confusing not knowing who to ask the cup of sugar from and who will meet them when mowing the lawn.
Many heterosexual couples are supportive of the freedom to marry.
When children raised in same-gender households are part of a discussion, many heterosexists argue that children need both male and female role models. For a long time, queer families have been living in society without timeworn roles. As long as we were second class, or the law reinforced the heterosexist perspective of superiority, the majority population did not have to question gender roles. But the Baker decision gave legal bite to our contention that we, as gender-ambiguous people, are a valuable part of the human race and should be treated equally. We are excited to be in families that function well without typical gender roles, with children or without.