[Reposted to fix comments, which weren't working originally.] Prof. Glenn Reynolds (InstaPundit) asked whether this would violate Massachusetts antidiscrimination law:

Harvard University has moved to make Muslim women more comfortable in the gym by instituting women-only access times six hours a week to accommodate religious customs that make it difficult for some students to work out in the presence of men.

Men have not been allowed to enter the Quadrangle Recreational Athletic Center during certain times since Jan. 28, after members of the Harvard Islamic Society and the Harvard Women's Center petitioned the university for a more comfortable environment for women....

Prof. John Banzhaf has put out a press release saying that it probably does:

... In 1998 a female weight lifter in Boston was awarded $5000 when she was denied admission to a male-only section of a gym which had a separate gym area for women. The Massachusetts Commission Against Discrimination made the ruling despite arguments that separate weight-lifting areas were necessary to prevent "sexual harassment," and a finding that it did in fact tend to reduce sexual harassment. [Hassan and DiCenso v. City of Boston, et al., 20 MDLR 83]

Just a year earlier Superior Court Judge Burnes ruled that a "women only" health club violates Massachusetts' public accommodation statute by refusing to admit men, and could not justify its policy on privacy grounds. [Foster v. Back Bay Spas, d/b/a/ Healthworks Fitness Center, Suffolk Superior Court No. 96-7060 (1997).]

Although the legislature responded by exempting some health clubs which are established solely for use by one gender, that exemption does not appear to apply here because the gym is used by both genders together during most times of the day, and because Harvard receives public funds. In the statutory words:

"however, that with regard to the prohibition on sex discrimination, this section shall not apply to a place of exercise for the exclusive use of persons of the same sex which is a bona fide fitness facility established for the sole purpose of promoting and maintaining physical and mental health through physical exercise and instruction, if such facility does not receive funds from a government source." [ALM GL ch. 272, § 92A] ...

A couple of quick thoughts:

I may be mistaken, but my sense is that the relevant statute wouldn't apply to Harvard if the exercise facilities are inded open only to Harvard students, faculty, staff, alumni, and some family members. The statute states that, "A place of public accommodation, resort or amusement within the meaning hereof shall be defined as and shall be deemed to include any place, whether licensed or unlicensed, which is open to and accepts or solicits the patronage of the general public" (emphasis added), and Harvard-only facilities aren't aimed at "the general public." Cf. Haskins v. President and Fellows of Harvard College, 13 Mass.L.Rptr. 691, 2001 WL 1470314 (Mass. Super. 2001) ("Although Harvard accepts applications for admittance from the general public, it admits only a small fraction of applicants. The unsuccessful majority (like the rest of the public) is then excluded. Thus Harvard is not a place of public accommodation within the meaning of the statute.").

If there's some statutory text or caselaw I may have missed on this, please let me know.

UPDATE: Thanks to Wallace Forman for the pointer to what seem to be the eligibility criteria for Harvard recreational facilities; the original version was more tentative on the criteria, and also omitted the possibility that family members might qualify (a possibility that does not affect the analysis, I think).