Your statement at one time had one meaning -NOW -it could mean anything -in other words it is now meaningless, as it is vague... e.g. one can be monogamous with children, animals, and adults of the same sex or not the same sex with or without a true marriage...
Aside from the illegality of some of your examples, faithful monogamy still would work to prevent the spread of AIDS.
This is a good example of evolution at work.