What do we mean when we say that a certain human (or even non-human animal) group X is marginalised (in a context C)?
The expression is ambiguous.
It could be a descriptive claim, meaning that
A. A (generic) X is at a disadvantage with respect to a (generic) non-X (most commonly a hegemonic group Y) in a common context C.
For example, to say that women are marginalised in Mexico might mean that women make less money that men, are less represented in positions of power, receive less education and health services, etc.
But it could also be an explanatory claim, meaning that
B. A (generic) X is at a disadvantage with respect to a (generic) non-X (most commonly a hegemonic group Y) in a common context C because she is X (and not Y).
In other words:
B. A (generic) X is A-marginalised (with respect to Y and C) because she is an X.
[From now on I am going to obviate saying that these claims must be read as generic and with respect to a hegemonic group in a common context. Also, in theses B onwards, marginalisation will mean A-marginalisation]