Marriage isn't even a part of the question, it is, should every person have equal rights in general.
100 years ago, the answer was Yes
Today, the answer is Yes
100 years later, the answer will still be Yes
This is because it is fundamental that every person should have equal rights in general.
It is just that at some points in time, people have made exception to this rule for whatever reasons, intents or purposes.
e.g. gay marriage, slavery...etc.
However, it is still right (at that time) that every person should have equal rights, they just hadn't recognised it yet.
Hence, as soon as it is recognised, society will change in order to follow this moral ideal. This is why we've abolished slavery and started talking about gay marriage, because we see that those are SUBSETS of our moral ideals.
There are no changes in morals, we all believe that people should be given equal rights, however, yes, there is a change in consensus as people come to understand more of the SUBSETS of their moral ideals.
Hence, morals do not change.