Why don't we call girls boys?
Marraige is a union between a man and one or more women.
If you 'marry' a horse with an as s the result is neither. Why would anyone try and say it is anything but something different?
If you want to join two or more of the same things, it is simply more of that same thing and not a marraige.
You can marry two dissimilar ideas to create a new idea. This is marraige. Rice plus rice is still rice. This is Not marraige.
Bastardizing the meaning of a word is not necessary to accommodate unions or joinings of similar things. There is no such thing as a marraige of two identical things. A new or different word may be needed, but I don't want the meaning of the word marraige to be unnecessarily changed.