Benefits for marriage, such as taxes, health benefits, etc were designed to reward a stable marriage. Studies have shown that, in a functioning, stable marriage, that both the parents and their children tend to perform better in various aspects of their lives and provide more back into society. Adults tend to be stable and efficient workers in the workplace, children perform better in school, grow up better behaved. People live longer, take fewer dangerous risks, have lower depression rates and higher happiness rates, and are healthier in general.
While this is "in general", we recognize there are exceptions.
On the other side of the coin, gay partners tend to be more promiscuous, and tend to be far less monogamous. While there may be a few exceptions where partners truly are committed to each other, the greater majority of these partnerships tend to be less stable.
A properly functioning marriage is a great benefit to society as a whole. The real shame of the matter is that current culture has made it acceptable for people to divorce or re-marry. To me, divorce is an excuse to not working out your issues. There is no such thing as "irreconcilable differences" or whatever babble lawyers throw out as reasoning for divorces. It's too easy to say "I give up". You harm both yourselves in the midst of the proceedings, and you harm your children, who don't understand why Mom and Dad can't work things out and more often than not, needlessly carry burdens on themselves. Marriage was designed to last for life.
This is why conservatives and evangelicals support hetero-sexual marriage and unions. While they/we are not perfect ourselves, we recognize success rates are higher in this norm.
If these views makes you believe me/us to be bigoted or ignorant, i think you need to reevaluate your definition of those words, rather than resort to name calling when someone disagrees with you.