GAW: You started your post by stating that you cannot explain the positions of others then ended it by stating you see no reason not to extend the “rights’ of marriage to same gender couples.
So let’s try and define things.
1) no one was denied the right to marriage, any man can marry any woman.
2) There are those who choose for whatever reason not to enter a marital relationship, in a democracy you cannot force someone to do anything (except buy health insurance) and therefore they had the right to stay single or enter other types of relationships.
3) Due to the fact that some people did not like what marriage was they decide to change it.
Sort of like if I do not like what a table is I try and change it to a chair.
As such they began to insist that a loving relationship between two same gender individuals is also a marriage.
Now factually, scientifically, and inarguably any way you want to put it there are many huge difference between the relationship between a man and a woman and tow individuals of the same gender.
1) Biologically the relationship is extremely different
2) emotionally they are extremely different
3) The side effects are extremely different, on can have kids one cannot.
However these individual decided that none of above characteristics are what define the term marriage rather it is the presence of “love’ and any two individuals in a loving relationship are married.
If the only requisite for marriage is “love” then again why is a mother and daughterer not married?
Why are two best friends not married?
why are two close siblings not married?
In my mind the answer is quite simple.
Love is an important part of marriage but it is not the only part there are many unique aspects of that particular relationship.
The combination of all these factors are found only in the relationship between a man and a woman and it is that unique relationship that is marriage.
Hence the secular “argument” against same gender marriage is essentially.
It simply is not marriage. It is relationship it is many things but it is not marriage.