Actually, it is only required that you REQUEST the information from the applicant. While it is good practice to get the information, it is not a violation if you don't.
202.13(a) Information to be requested. A creditor that receives an application for credit primarily for the purchase or refinancing of a dwelling occupied or to be occupied by the applicant as a principal residence, where the extension of credit will be secured by the dwelling, shall request as part of the application the following information regarding the applicant(s):
(3) Marital status, using the categories married, unmarried, and separated
202.13(b) Obtaining of information. Questions regarding race or national origin, sex, marital status, and age may be listed, at the creditor's option, on the application form or on a separate form that refers to the application. The applicant(s) shall be asked but not required to supply the requested information. If the applicant(s) chooses not to provide the information or any part of it, that fact shall be noted on the form. The creditor shall then also note on the form, to the extent possible, the race or national origin and sex of the applicant(s) on the basis of visual observation or surname.