I would agree that the unnatural thing the women are doing in that verse is probably them not bearing children, in whatever fashion they are avoiding it.
The "unnatural thing" that women are doing in v26 is the same thing that the men are guilty of in v27, which is "abandoned natural relations with women and were inflamed with lust for one another.". So basically it is wrong for women (and men) to exchange hetrosexual sex for homosexual sex.
The thing that complicates this, from a polygamist angle is that this says nothing about a man and two wives at the same time and even two women doing things to each other to please the man as it is not turning away from man, like this verse is describing.
The best answer always seems to end up being for every man to decide for their own family and for him to work it out with his owns wives and keep it to yourself and not judge others for having a different opinion on this controversial subject.