I agree with everything Cindie said, and I'd like to add, What about her female partner's veiws? You said she's been living with a woman for quite a while. Is she OK with her lesbian lover suddenly being in a relationship with a (married) man? How does she feel about this sudden huge change? Is she poly? Does she want kids? Does she want your h's gf to move out, and in with 2 relative strangers? Does she want to be a mom too? Will she be involved, or is their relationship struggling to the point they on the verge of breaking up? Etc etc
I'd say you both need to meet with her as well and hammer out just the adult relationship variables here thoroughly and responsibly, long before an innocent child is brought into the mix.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37