OP, i have an opinion formulated already but before i give it, i will propose some questions to which i have not noticed you'd given certain background information already.
Is the male partner in your relationship the biological and/or practical father of your grown children? If not, does he have any children with anyone else? If so, has he been in their lives as their "father figure"? If not, does he want that?
What about the other female in this relationship? Does she have children? Does she WANT children (early 40's hmmm biological clock timing out but shit happy accidents happen tra la la)? Have the three of you talked about "what happens IF" she gets pregnant?
My opinion is that your male partner either consciously or subconsciously WANTS to get this woman knocked up because they want a baby together. I would even go out on a limb and suggest that they have talked about it between the two of them and said "wellll if it happens it was probably meant to be, let's just hope for the best and cross that bridge when we come to it." Although they may have convinced themselves that this is only a way to not have to use condoms (let's face it - who LIKES using them things unless they HAVE to?), once she is pregnant it will be a fait accomplis, and "ohhh i just CAN'T have an abortion after all".
And the decision will be made. The decision HAS been made; you are just trying to figure out what it is.