She gave birth but was sent to the colonies afterwards, so perhaps that has reestablished her as an unwoman.
In the book I think they watched a video of the colonies where feminists like June's mum were sent to the colonies and called unwomen. I don't think it's just for the infertile, but for those who transgress in some way too. The women in the colonies were all unwomen for one reason or another I think.
But perhaps Isaac was using unwomen like some men use cunt or feminazi or bitch. He was calling her the worst thing she can be in Gilead.
Rita had his measure when she said he could break her jaw. He would and he'd enjoy it, because he's a toxic man enjoying his place above the women.