Looking at it again, I'm even less sure that Mab is actually saying she can't marry (in general), as opposed to can't marry
Lara.
As with most Fae things, precise wording is likely important.
You assume a monogamous marriage but the Sidhe are old, old style and pre christian. They might not have that clause.
Lara certainly doesn't want it.
Oh absolutely there would be no expectation of fidelity for either Winter or White Court. It's more a matter of, I thought Harry's oath to Mab was "absolute" enough that it might be incompatible with later marriage vows unless they were written specifically to deal with that.