Marriage: What's the Problem?

So what exactly is the problem with the issue of same-sex marriages? Why is the Catholic Church against it?
First, let us define "marriage." Marriage is a sacrament of the Church, and is solely defined as the union of one man and one woman. Bearing children is also a part of marriage, as defined in Genesis 2:28 "God blessed them and God said to them: Be fertile and multiply." Same-sex couples are neither, and since the Church is pro-creation, it is unable to support same-sex marriage.
However, people will inevitably choose to live that lifestyle. Who are we to deny them the same benefits that heterosexual couples enjoy? I mean, something as basic as sharing a health insurance plan or claiming social security is important in their life. But, either way, Marriage is a sacrament and must be protected. What advocates for same-sex marriages are looking for is a Civil Union with the same federal benefits as traditional Marriage. The problem is that we use the two terms Civil Union and Marriage interchangeably, when they are completely different.