I do agree that single mothers should be able to join GLO but because parenting in itself requires a lot of time and commitment, they should make sure they'll have the time to do both. I am a single mother and my lovely Sorhors welcomed me and my daughter with open arms and have been a great support system. My daughter now has lots of "aunts" - probably more than she'd like.
As for pregnant women, I think they should wait because pregnancy is stressful all by itself.