At most colleges, having an overnight guest of a different gender than yours is totally fine as long as your roommates agree to it. Northwestern’s student handbook [url=<a href=“http://www.northwestern.edu/student-conduct/shared-assets/studenthandbook.pdf%5Ddoes”>http://www.northwestern.edu/student-conduct/shared-assets/studenthandbook.pdf]does not explicitly mention anything about student gender/url, which is usually a good indicator that gender is NOT an issue.
There are still some colleges that do care about this - for example, at my undergrad college (Spelman) male overnight guests were not allowed, although I’m not sure if they have changed that policy (I graduated almost 10 years ago).