Not only do biological mothers get more leave than fathers or adoptive parents, but such policies disproportionately affect LGBTQ+ employees, the report says, adding such "unequal policies not only reinforce outdated gender stereotypes, they are often in clear violation of the law."

The number of companies providing paid paternity leave is actually falling, declining 5 percent between 2010 and 2014. In addition, the report finds 17 companies leave out all or some of the fathers and adoptive parents in their workforce, and eight top companies provide no paid family leave to any dads or adoptive parents (the eight are Albertsons, CVS, Gap, Kroger, Publix Super Markets, Staples SuperValu and Walgreens).

Related: JPMorgan Chase is biased against dads, ACLU complaint says

Seven companies — Amazon, Apple General Electric, JPMorgan (for secondary caregivers), Procter & Gamble, Walt Disney Company and Wells Fargo — provide significantly less paid family leave to dads and adoptive parents than they do to birth mothers. "Significantly less" is defined as greater than 8 weeks' disparity.

(Editor's Note: After publication, Wells Fargo contacted BenefitsPRO with the following statement, courtesy of Diana Rodriguez, Wells Fargo senior vice president of corporate communications: "Wells Fargo's policy is in fact gender neutral and we give equal paid leave following birth or adoption. We allow a team member to identify as either the primary or secondary caregiver, regardless of gender — 16 weeks of fully paid leave is provided for a primary caregiver, and four weeks is available for a parent who is the secondary caregiver.")

And nine companies provide paid parental leave to higher-income adoptive fathers or adoptive parents (such as those who work in corporate offices), but for the lower-paid majority of their retail workforce, policies vary but are far more restrictive. At those nine companies, here are the parents who are excluded:

• Amazon: part-time dads and adoptive parents

• Darden: hourly dads and adoptive parents

• Marriott: part-time dads and adoptive parents

• McDonalds: hourly dads and adoptive parents

• Nike: part-time dads and adoptive parents

• Starbucks: field employees who are dads and adoptive parents

• Toys "R" Us: hourly dads and adoptive parents

• Walmart: hourly dads and adoptive parents

• Yum! Brands: field employees who are dads and adoptive parents

Only 10 provide equal paid parental leave to all new parents.

Related: What bipartisan compromise on U.S. paid leave might look like

While "most Fortune 500 companies now pride themselves on offering equal benefits to their LGBTQ+ employees and have evolved many of their policies toward inclusivity," the report says, "since LGBTQ+ people are more likely to be adoptive parents or dual-father households, unequal paid family leave remains one striking area in which major companies are leaving these employees out."