MARRIAGE CANNOT BE REDEFINED

To be fair many words have changed their meanings or have had more than one meaning at times... So I think this point should be 'MARRIAGE SHOULD NOT BE REDIFINED' imo. Language categories that is to say definitions act according to the concept of Family Resemblance. https://en.wikipedia.org/wiki/Family_resemblance
Or in other words language has a tendency of veering towards categories of things with many similarities rather than one essential feature. By the way, the guy who came up with this concept was an autistic philosopher who wanted a precise language for everything but then gave up.

Personally, the problem that I have with all this is not the semantics per se but I don't think that the state ought to have the power to confer benefits to any pairing of people- be they man and woman or man and man or whatever.

I think the problem with the redefinition as you put it is that it elevates homosexual relationships to the level of heterosexual relationships when in your view (I still need to see some evidence for this) homosexual relationships are worse at raising children. To be honest I don't think this is a huge problem because even if homosexual relationships are worse at raising children they will always be a minority. Same-sex marriage is a Stalingrad imo. The real problem is the collapse of normal marriage- and in my opinion, a large part of that is due to state intervention in normal marriage providing an active incentive to break up marriage.