This is all said in jest, but as a former WNBA fan with Minnesota Lynx season tickets for several seasons, I think it's a legitimate concern.

First, the league has always marketed itself as very liberal to its heavily lesbian audience-- and I don't say that as any sort of attack, I'm merely stating a fact: a huge chunk of the players and punditry are open lesbians, and probably half of the adult attendees when I used to go to the Lynx games at the "Target Center" were women with short hair cuts, holding hands. There is zero possibility the league could say no to letting trans people in: it would be political and financial suicide.

Second, I remember the fawning idolatry the league's punditry had for Lisa Leslie: she was the league's dominant center at 6 ft 5 in, 170 lbs. They loved her in some part because she could (barely) dunk. If she was that dominant, imagine introducing a 7 ft 2 in 250 lbs former-NBA player into the league. How would the league's "girls rule, men drool" commentary react when said player was putting up fifty points and fifteen blocks per game? Would they just go along with it and pretend that the person was a biological female? Would they rebel and feel like it was an invasion?

Third, it wouldn't even necessarily need to be an NBA-caliber player: in football (soccer), boy's high school teams routinely beat women's professional sides-- there is that big of a descrepancy between males and females. All it would take to destroy any sense of competitiveness in the WNBA is for even two or three trans people to go in: those two or three teams would be so dominant, they'd be impossible to stop! And if you think that's an exaggeration, think of how many WNBA games are decided by just a few points: having "Lisa Lesie x2" would virtually guarantee a championship.

The only way I could see it working would be to have a requirement that the trans individual has been on hormones: then they could very well have lost height and muscle mass. But based on all of the precedent in other sports, that is almost certainly not that route things would take.