That is a tricky situation - thanks for coming here and sharing it with us.
Polyamory is about love - it may well be about sex too, but primarily it is about love. Was this not something that you guys discussed when you were talking about getting into this?
Let me see if I understand your situation correctly - this colleague of your husbands, who is married and lives remotely, wanted somewhere to stay during the week. Your husband suggested that she stay with you guys.
First question - did you husband and her have some sort of romantic or sexual connection before she moved in, or was she just a colleague and this developed after she moved in?
Once it got sexual, by whatever means, you made a deal with them that she would get pregnant because you wanted a baby. This sounds like the agreement was that if she got pregnant, she would allow you and your husband to legally adopt it - is that right?
Second question - did everyone involved (the two of you, and her and her husband) all agree to this? Was anything legal put in place to make sure that she couldn't just change her mind and leave with the baby?
Now they tell each other that they are in love with each other, but he is telling you that he is lying to her about that, and only doing it to keep her happy. Either way he is lying - either to you or her. Neither sounds particularly healthy to me. Part of poly is being open and honest and he isn't. He needs to change that if this is going to work in any way.
Third question - did you have a discussed agreement that this would be sex only and that no love would happen? Or was this just assumed? If it was discussed, was it discussed with just the two of you, the three of you, or all four? If it was explicitly discussed, then them falling in love is breaking your agreement. If you assumed it without a discussion, then this points to some real communication difficulties between you and your husband.
He claims that the reason he isn't being honest with her is because if he didn't say he loved her, she would go out and have unprotected sex with other men.
Fourth question: Have you negotiated some sort of "closed' relationship agreement with her/them? If not, then she can certainly go out and have sex with (and fall in love with) anyone she chooses - she's poly, after all. No amount of him telling her he loves her will change that. If you have negotiated a closed policy then you have an agreement in place that she won't, so him telling her or not won't make any difference.
If you don't feel like doing the sexual threesome with them, why do you? Why put yourself through something that you don't really want or enjoy?
Please correct me if I am making wrong assumptions about what has gone on - that's the reason I am asking the questions, so that we can have a better idea of what is going on and what has gone on.
"Listen, or your tongue will make you deaf." - Native American Proverb