Presumably if they need to ask the transmen to sign a waiver absolving the RFU of any risk, then why would they not need the women's team to sign a waiver as well.
It is not beyond the realms of possibility (a la Maxine Blythe, champion women's cricketer who also plays on a men's team) that a transman playing on the men's team could meet a male player that is on both the mens' and the women's team, yet the female team members will not be asked to sign a waiver.
As research has proved, any performance depletion caused by a minor reduction in testosterone can be mitigated fully by extra training, so there would be no obvious reason that a male player could not play on both the men's and the women's teams.
I guess the RFU could try and discourage this from happening by making the rule that you have to stick to the 'identity' you have chosen (like cycling appears to have done) and if you register as a female player, you have to remain exclusively a female player.
However, this then creates the scenario where a male player is banned from playing on a male team. To work this through, a male player could register as female to play on the female team. That female team could end up disbanding because not enough women are happy to play with a male.
That male is then stuck with no club to play for even though there is a male club that he should be able to play in were it not for the fact that he has another 4 years (or whatever the rule says) to run on his 'female' registration. Would the RFU then allow this person to reregister to the men's team on the grounds that otherwise they can't play at all? If this is the case, then it makes a complete mockery of men's and women's teams - they may as well make all rugby mixed sex.
The RFU will work themselves into a massive mess with this one. As pp says, it is discrimination on grounds of sex - where some males are allowed to do something that others are not. However, it takes time, money and will to bring a prosecution and I'm not sure many men who identify as men are going to be willing to sue RFU because they want to play on the women's teams.
Then there is the bizarre asymmetry where a female born player on the male team is asked to sign an injury waiver but a female player who is asked to play against a team with one or more male people is not.
This whole situation with the RFU makes me really sad - in my mind rugby was one of the last bastions of old school fairness and sporting behaviour - there are extremely strict safety rules and I thought that they were better than this. I have always been a rugby fan (I dabbled a little in playing at university but I didn't like getting bashed and am too small but the social life was fun) but I did like the ethos of the game. Now I'm not sure.
The RFU probably realises that it is the pinnacle of dangerous contact sport. If they ignore the safety of women and allow male people to compete with them, then it opens the doors for every other sporting body in the country/world to point at rugby and say 'well, if they can do it, it is fine for us to do it'.