U.S. Federal Regulations
An employer having only unisex, multi-person restrooms is a violation of OSHA regulations. Sex-specific restrooms are required, at least for the employees, unless the restrooms are only single-occupancy.
While, as another answer mentions, California has a law authorizing cities in California to require restrooms to be gender-neutral, such laws would be unenforceable as preempted by federal law unless/until the OSHA regulation is changed.
(For those not familiar with U.S. regulations, OSHA is the federal Occupational Safety and Health Administration, which regulates workplace safety. Its regulations apply to the entire United States.)
29 CFR 1910.141(c)(1)(i) is the relevant regulation here (emphasis mine):
Except as otherwise indicated in this paragraph (c)(1)(i), toilet facilities, in toilet rooms separate for each sex, shall be provided in all places of employment in accordance with table J-1 of this section. The number of facilities to be provided for each sex shall be based on the number of employees of that sex for whom the facilities are furnished. Where toilet rooms will be occupied by no more than one person at a time, can be locked from the inside, and contain at least one water closet, separate toilet rooms for each sex need not be provided. Where such single-occupancy rooms have more than one toilet facility, only one such facility in each toilet room shall be counted for the purpose of table J-1.
There is an explicit exception to the requirement for the restrooms to be sex-specific for single-occupancy restrooms, but there is no exception for restrooms designed to be occupied by multiple people as you've described.
This particular regulation applies only to restrooms that are available to employees (as opposed to those exclusively for use of patrons.) Requirements for those exclusively for use of patrons are set by state-level plumbing codes.
Maryland Plumbing Codes
While the OSHA regulation above does not apply to restrooms for use only by patrons, Maryland's plumbing codes require separate facilities for each sex for those, too, with a few exceptions.
Section 403.2 of Maryland's plumbing code (emphasis mine):
403.2 Separate Facilities
Where plumbing fixtures are required, separate facilities shall be provided for each sex.
- Separate facilities shall not be required for dwelling units and sleeping units.
- Separate facilities shall not be required in structures or tenant spaces with a total occupant load, including both employees and customers, of 15 or fewer.
- Separate facilities shall not be required in mercantile occupancies in which the maximum occupant load is 100 or fewer.
- Separate facilities shall not be required in business occupancies in which the maximum occupant load is 25 or fewer.
403.2.1 Family or Assisted-Use Toilet Facilities Serving as Separate Facilities
Where a building or tenant space requires a separate toilet facility for each sex and each toilet facility is required to have only one water closet, two family or assisted-use toilet facilities shall be permitted to serve as the required separate facilities. Family or assisted-use toilet facilities shall not be required to be identified for exclusive use by either sex as required by Section 403.4.