One, the state co-opted a religious institution (marriage), and started giving benefits for it. (Doesn't matter why, though I suspect to encourage population growth or an increasing population of tax payers.) This crosses the separation of church and state principle. Government should not turn a religious institution into a civil one. (You know, like a day of worship...) We don't live in a theocracy, so it doesn't matter how your religion defines marriage. It is a religious institution and the government should have no say in the matter.
Two, if you want the government involved in marriage, and you're going to give rewards (tax benefits, asset transfer, etc.) to two people who enter a coupling contract, you can't arbitrarily disallow others to enter that same contract and receive those same rewards. You're giving one class of citizen a benefit that is out of reach of another. It's like giving a benefit to right-handed people only, or whites only. It is immoral and unethical and is anathema* to liberty. (*Oh, the irony!)
Three, you have to understand that if you give tax benefits to civil unions/marriages of two people contracting to commingle assets (and that's really all it is in the perception of the state), you have to also extend those benefits to three people contracting to commingle assets. As well as 4, 5, or 6 people... So long as all are consenting adults, it is unethical to deny benefits to this category of contracted union (bigamy, polygamy) while allowing those same benefits to another classification of contracted union (monogamy).
The solution to all this is: the government should get OUT of the "marriage" business.