Oh dear, this is appalling. Yes, they better slow the f down.
Moving her in after 2 months is silly enough. Knocking her up and bringing a new person into the world after such a short period would be bad enough if he were single. Doing that knowing you've got a bit of baby fever yourself makes it worse. Not knowing her well enough, and about her relationship with her female SO also makes it even more sketchy.
I know you are afraid of losing him, but this behavior and lack of forethought can not be tolerated. If you're the only rational adult in this situation presently it is up to you to insist he put on the brakes and do some serious thinking, evaluating and communication with both her and you. Sounds like he's got a bit of a mid-life crisis going on... a man in his 40s feeling all twitterpated and flattered to be desired by a woman in her 20s.
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