Woww.. Now you're just going wild, Partysane.
"It is in no way your right to interfere with the live of others as long as they again do not infringe the liberty of anyone."
I agree, but hold the phone there. Being against polygamous marriage doesn't "interfere in a polygamous relationship" per se. I'm perfectly fine with five people living together and all of them being intimate.
What I'm hesitant about is, for example, rewarding that with a tax break that I, as an individual not living intimately with five people, don't have. I'm hesitant about having these five-way couples adopt a child, that I will have a certain responsibility for as a citizen (my taxes will pay a good portion of the child's education).
I agree that as a general rule, whatever doesn't hurt others should be allowed, but in this particular debate, I believe there's more to it than that..
Regarding the tax breaks themselves, there's good reasons to give them to heterosexual couples who decide to stick together. Long-term relationships are known to be productive environments for children to become good citizens. And children are needed to have a steady-state society, which in itself is something that is beneficial to the citizens themselves (societies with politics that strongly limit the number of children born, such as China, and that allow lots of children to be born in a short period, such as the Gaza strip, experience serious demographic problems).
In other words, marriage as a financial stimulus to heterosexual couples is justified. The question whether that insitution of marriage should be extended and if so, to what point, is not at all as self evident as you claim it to be.