Only two months and talking babies?? BIG RED FLAG.
I don't think your husband is thinking with his head here... NRE perhaps? And your question of why with her and not with you is VERY VERY Valid... children are a huge committment - why after only 2 months is he willing to change his mind on something he has been so adamant about for so long?
I think NRE is a huge issue here.
*mono women in poly relationships? - Sage and Mohegan are the two on here that immediately pop into my head... I would recommened their blogs and threads
good luck and welcome