“Having signed this form, married women are assumed to have received their spouses’ concurrence.”

Or, a little less clunky:

“Married women who have signed this form are assumed to have received their spouses’ concurrence.”

“By signing this form, married woman indicate that they have their spouses’ concurrence.” Or “Married women who sign this form indicate that they have their spouses’ concurrence.” Writing the statement like this keeps the focus entirely on the people who’re signing the form.

You could say “Married women who sign this form are assumed to have their spouses’ concurrence.” This is grammatically correct, but the passive voice “are assumed” again begs the question, who is assuming? The context may answer, but use the active voice if this is writing where style matters. If it’s an application or something like that, the second may be preferable.

“it is supposed” is passive. An active voice like “we assume” or “married participants must have their spouses’ agreement” would be better here.

If you’re writing this, think about what you mean and find a way to use active voice; It’s generally much less convoluted and more understandable. It’s also preferred now to say “married persons” or “married people” rather than the more old-fashioned “married women.”

To address another point, the it in “IT is supposed” does NOT modify any word in the sentence. It does indirectly need to match the number of the “supposed” which is singular. That’s the tricky thing with passive voice.

Two examples:

“It is assumed that…”

“It is requested…”

Again, the IT matches the number of the “assumed” and “requested” which is always singular.

A better way to state it would be:

“We assume that…”

“I request…”

Then the pronoun matches the plurality of the real subject.

Hope this helps!

Or you can also try: “It is supposed (or assumed) that married women who sign this form have their spouses’ concurrence.”