Firstly, there were female members of the UVF and UDA. You're right in that they were very, very, very rare. Off-hand I can only think of two. Johnny Adair's wife was a member of the UDA and took part in UDA firing parties and other operations. There was a female UVF member in Belfast during the 80s I can remember reading about somewhere.
Secondly, the ideology and nature of the organisations would mitigate against it. The IRA consciously modelled itself as a revolutionary, socialist, national liberation movement. Consequently, it fell into the perception of seeing women as equal participants in struggle, as supporting the liberation of women and so on. National liberation movements in Africa, Europe and Latin America accepted women in their ranks.
We shouldn't exagerrate either. While there were hundreds of female IRA volunteers over the last thirty years they never made up anything but a minority. The IRA was a male dominated organisation, but unlike Loyalism encouraged and attracted female militants. As I write this Maireád Farrell's photograph is right in front of me, but there were many more less high profile.
Loyalism on the other hand is essentially a reactionary ideology, quasi-fascist in nature with a focus on opposing change, maintaining the status quo and thus hardly a place where culturally women would be seen as part of their struggle. Bear in mind too that Loyalist paramilitarism never had the level of support in its community that the IRA did, and that women who might have wanted to take up arms against the IRA could, and did, join the RIR or the RUC where they could be respected members of their communities.