You don’t need a tag for it, or any other form of marriage. You are supposed to use the TYPE tag for those.
2 TYPE common
A marriage is a marriage, but there are different types, so record them as such.
Tags like _COML were generally invented by software companies who didn’t understand the GEDCOM spec, and felt it was “easier” to just invent more tags. So now we have a standard that has an unjustified reputation for not working properly and is barely usable as a data transfer mechanism because of it.
TYPE currently covers Religious marriage, Civil marriage, Common-law marriage, Registered partnership, and Unknown. I believe that covers most needs.