Late reply to Steve, probably will get lost in the shuffle but that's fine.
Maybe there should just be a belt system. Rousey's (as just one example) has a black belt; and, iinm, there's not a different ranking system for women. Wasnt't there a famous female judoka who passed away not too long ago? At any rate, with rankings, only issues of weight classifications would matter.
Yes, Keiko Fukuda of the SF Bay Area passed not long ago. 10th dan and the only female ever to achieve that rank. But, belts don't make equal players. Judo is segregated and that's a good thing (once they are out of the kiddie pools, as you rightly mentioned).
When I was competing in college I got to do some heavy randori with a female world champ and silver medalist Japanese judoka who weighed about as much as me. I could take her. I think I was just a freshly minted shodan, or maybe not even that. Because strangulations are legal from preteens and up, and elbow and shoulder locks are allowed in black belt pools, it's already dangerous enough. I don't really remember any shiai where someone didn't either get something broken or get choked out for too long. The application and release of kansetsuwaza in judo is very different from BJJ. It goes on very fast, very hard, and you don't stop until the ref stops you--this is generally a bit too late.
I don't have a major issue with women competing against men if they know what's up, but the other way around seems unfair.
Also, the Kodokan has a separate women's division and their black belts had a white stripe through them. Very interesting.