Jamieson, Fausset and Brown on 1 Corinthians 7:36

36. behaveth ... uncomely--is not treating his daughter well in leaving her unmarried beyond the flower of her age, and thus debarring her from the lawful gratification of her natural feeling as a marriageable woman.

need so require--if the exigencies of the case require it; namely, regard to the feelings and welfare of his daughter. Opposed to "having no necessity" (1Co 7:37).

let them marry--the daughter and her suitor.