Many of those rules and requirements are specifically for the protection of children and would be silly applied to a same sex couple.
Many same sex couples enter into a marriage with preexisting children. Gay people are not incapable of siring children. They just can't sire them with each other... Many straight people marry and find they cannot make a child either and thus they search for alternative ways to bring a child into their relationship in the exact same way gays do. The laws can amend the way marriage is applied without changing the word marriage into civil unions.
As for the legal definition of a man and woman. Some women are masculine and though married to a man they take on the male role in a marriage. Some men stay at home cook, clean and take care of the children and are effeminate. Today that is not frowned upon but in the past in the ideal world of marriage that you seem to subscribe to return to they were persecuted in society for doing so. That made for an unhappy environment to raise children.
So should their marriage be called a civil union because they do not fit the precise mold where roles are reversed. Just as in many gay relationships there are often male and female roles. Not all gays even want children just as many heterosexuals opt to not bring children into their union.
There is a higher divorce rate because in the past divorce was so socially forbidden. That instead of divorce couples were forced to stay in an abusive relationship. The children suffered horribly in those situations. Just as gays were persecuted in the same traditional society that they remained in the closet. Male and female gays were often so unhappy that they battered each other in a marriage situations...
Often suicide or murder of a spouse resulted. How many wives poisoned their husbands in the old days because they found they were gay? How many husbands beat their wives because they couldn't deal with their own inner gay person. This was why marriage was often referred to as a ball and chain. Wife swapping was much common in the old days. Today people just divorce and adjust their lives to more closely fit the person they really are. Isn't this better?
I do think however that changing the legal definition to accommodate you and your partner would further change the attitude about marriage and even more people wouldn't bother than already do.
Homophobia is an "attitude" too. It was precisely the social stigma and attitudes that led to all of the repressed marital troubles in our societies past. The religious right forcing prolonged marriage on people after the love and tenderness was simply gone... Thus in turn this forced children to remain in hate filled parental situations with no hope of escape.
Today people feel more free to divorce and that is a good thing... Perhaps marriage also encompassing gays will allow heterosexuals to be even more open about where the really want to be. Bad marriages could then dissolve more easily and morph into a more loving situation for the children involved. As I said, two households give the children two choices rather than only one.
Some with "attitudes" feel sorry for children raised in gay marriages but is that any worse than children raised in an abusive heterosexual marriage? Especially when the children in the gay family are often raised in a loving and tolerant atmosphere in the gay couples household?