• The National Women's Soccer League allows private equity firms to take majority control of franchise teams, unlike other U.S. professional sports leagues.
• Sixth Street and Carlyle have each have each struck deals at soaring team valuations.
• "We really see institutional capital as a way to really infuse additional capital behind our assets," Jessica Berman, commissioner of the NWSL, told CNBC in an interview. 

Women's soccer is bringing private equity off the sidelines.

While other major U.S. sports leagues – Major League Soccer, the National Basketball Association, Major League Baseball and the National Hockey League – have allowed private equity investors to take passive, minority stakes, only the National Women's Soccer League has allowed these firms to take majority control of the economics. 

"We really see institutional capital as a way to really infuse additional capital behind our assets," Jessica Berman, commissioner of the NWSL, told CNBC in an interview. 

Sixth Street was the first to own a team in building out the San Francisco women's team, Bay FC, last year. At the time, the firm paid a record $54 million for the league's 14th franchise.

The second-ever such deal closed a few weeks ago, as Carlyle partnered with men's team, the Seattle Sounders FC, to buy that city's counterpart in the NWSL, the Reign FC. The transaction valued the Reign at $58 million – far beyond the $3.5 million it sold for just five years ago. 

As part of the deal, Sounders FC owner Adrian Hanauer serves as governor of Reign FC on the NWSL Board, while Carlyle's head of private credit, Alex Popov, serves as alternate governor. Popov said NWSL attendance, up more than 40% this year, is evidence of the momentum in the sport. 

"We're seeing that inflection point, we're seeing it for the right reasons," Popov said. "And there is a lot of things for all of us to do, to continue to have growth." 

The Reign's chief business officer, Maya Mendoza-Exstrom, said that the investment is matching the "intrinsic value of women's sports." She said Carlyle brings a depth of resources to the game, and that the firm can add value on the analytics side, as well as share expertise from its other portfolio companies. 

"So we've got to make smart choices," said Mendoza-Exstrom in an interview. "We have to run a sustainable business that is one where we are generating multiples of revenue over a very short period of time and putting a better product on the field."

This year, women's elite sports revenue is poised to cross the billion-dollar mark for the first time, according to Deloitte. Soccer represents roughly half of that figure, Popov said. 

The makeup of women's sports revenue is tilted more toward merchandising sales, ticket sales, partnerships and sponsorships, in contrast to men's sports revenue, which generates more from broadcast rights. However, in November, the NWSL signed a $240 million, four-year media deal – 40 times higher than the league's previous deal, although still a fraction of what MLS brings in from broadcast rights. 

Still, the tailwinds in broadcast are a key bullish thesis for many private-equity managers seeking to ink deals in NWSL, including Carlyle.

Disney CEO Bob Iger and his wife, Willow Bay, are nearing a deal to acquire Angel City FC at a valuation of $250 million, according to a person familiar with the matter. That valuation would shatter valuation records for a women's sports franchise. NWSL declined to comment on the potential Angel City FC deal.

As valuations continue to spin upward, it's likely the private-equity interest in the sport will persist. However, the league is still in try-out mode. 

"We are treading carefully because institutional capital is very different from the types of individual owners who have typically carried the stewardship of teams in local markets," Commissioner Berman said., "A lot of leagues are looking to our experience to see how that is going and whether there is a way to continue to moderate the way that institutional capital invests in sports."

The NBA, MLB, NHL and MLS allow private equity ownership of up to 30%. The issue is also being heavily debated in the NFL. NFL Commissioner Roger Goodell said in May that the league is making "real progress" on the issue and that there is a lot of interest in the space.