Northwestern: No problem having a guest of the opposite sex spend the night providing (per school rules):
“A resident may have overnight guests, limited to one guest per resident at any one time with approval of all roommates/suitemates. Overnight guest privileges are limited to a maximum of three consecutive nights and a total of seven nights per resident per quarter. A guest is defined as any person who does not have a contract to live in a given space.
A resident may host non-overnight guests at any time with approval of all roommates/suitemates. All guests who are not residents of the building must be escorted by a resident at all times and must be registered with the security monitor on duty.
No individual resident, overnight guest, or visitor may use the residence hall’s washroom facilities designated for use by the opposite sex.”
http://www.northwestern.edu/living/about-us/policies/policies-and-procedures/index.html
Most schools are pretty easy going about this stuff.