Just because 2 women love each other does not mean that shouldn't have the oppurtunity to be married. They are not equating a Gay marriage to a Straight Marriage. They want the equal oppurtunity.
Again, there is no denial of equal opportunity involved. Once marriage is defined however it's defined there is no lack of equal opportunity as long as everyone is eligible for it. Either one of the two women you describe above can, if they wish, enter into a marriage arrangement as it is defined just like anybody else can. For some reason we keep going full circle and back around to saying that a person deciding that they don't
want to enter into marriage as marriage is defined constitutes equal opportunity.
No matter how you define marriage, if you put any limits on it at all, you are potentially going to be denying someone recognition of some relationship or set of relationships that they might prefer. The only way you're going to create a situation where there is "equal opportunity" with respect to marriage is construed as meaning that one can have any relationship they happen to prefer recognized as "marriage" is to say there is no definition of "marriage" except to say that any relationship or set of relationships can be recognized as such.