It sounds to me, like a few things are going on for the wife in this relationship.
#1- She can`t have sex, and she is worried about the two of you having sex while she is feeling vulnerable. If this aspect is new for her (her husband having sex with another woman) while pregnant, and unable to participate, is probably not the right time to start.
#2- Having you suck off her husband, or hand-jobs, is a way for her to feel like she is meeting her husbands needs, by 'supplying' you to him. Especially if she has a fear of being cheated on. You, in a way, are a puppet she can dictate, and she doesn`t have to worry about him playing elsewhere.
#3 - Asking you to be their girlfriend. - I think she might actually be trying to offer you some table-scraps to keep you with them.
The basic thinking being ; ' That all women are just happy to be 'claimed' and have a boyfriend.' She thinks of this as a gift to you,..to keep you happy.
Regardless,....all of it,..is not good for you. You will need to talk to them both directly and tell them your take on what polyamory is, and what you want out of the deal.
Also,..please remember, it is ok NOT to be negotiable on some things. When someone says to you, ' We don`t want you to have sex with other men.' Yet, you cant have sex with THAT man either, it is your REASONABLE RIGHT to state ; " I respect that you are not ready to share your husband in that way yet, however, I will share my body where I please. I will however, make sure to keep sex safe, and be true to all parties.'
You don`t have to 'ask' permission from anyone, to control your decisions about your own body.