From the New York Times article (emphasis added):
In a cable to American embassies around the world, Secretary of State Rex W. Tillerson wrote that visitors who require a visa before entering the United States must then follow through on their stated plans for at least three months. If in that period they do something they failed to mention in an interview with a consular official — such as marry an American citizen, go to school or get a job — it will be presumed that they have deliberately lied.
“If someone comes to the U.S. as a tourist, falls in love and gets married within 90 days and then applies for a green card, this means the application would be denied,” said Diane Rish, the associate director of government relations at the American Immigration Lawyers Association. “This is a significant policy change.”
From the United States Foreign Affairs Manual (emphasis added):
9 FAM 302.9-4(B)(3)(U) Interpretation of the Term Misrepresentation
(2) Inconsistent Conduct Within 90 Days of Entry
(a) However, if an alien violates or engages in conduct inconsistent with his or her nonimmigrant status within 90 days of entry, as described in subparagraph (2)(b) below, you may presume that the applicant’s representations about engaging in only status-compliant activity were willful misrepresentations of his or her intention in seeking a visa or entry. To make a finding of inadmissibility for misrepresentation based on conduct inconsistent with status within 90 days of entry, you must request an AO from CA/VO/L/A. As with other grounds that do not require a formal AO, the AO may be informal.
(b) (U) For purposes of applying the 90-day rule, conduct that violates or is otherwise inconsistent with an alien’s nonimmigrant status includes, but is not limited to:
(i) (U) Engaging in unauthorized employment
(ii) (U) Enrolling in a course of academic study, if such study is not authorized for that nonimmigrant classification (e.g. B status)
(iii) (U) A nonimmigrant in B or F status, or any other status prohibiting immigrant intent, marrying a United States citizen or lawful permanent resident and taking up residence in the United States; or
(iv) (U) Undertaking any other activity for which a change of status or an adjustment of status would be required, without the benefit of such a change or adjustment.
(3) (U) After 90 Days
If an alien violates or engages in conduct inconsistent with his or her nonimmigrant status more than 90 days after entry into the United States, no presumption of willful misrepresentation arises. However, if the facts in the case give you reasonable belief that the alien misrepresented his or her purpose of travel at the time of the visa application or application for admission, you must request an AO from CA/VO/L/A.