I am mostly following, although it is very alien to me because I have very different feelings on the issue, but I am a bit confused by the part about morals.
Now, I understand that you are religious, if I understood you right, and that you think marriage is sacred (you're against divorce, you wouldn't want a child with a married man). But obviously you don't believe that relationships outside of marriage are immoral, since you are in one, and condoning a second one that is just starting.
So why is the idea of children different? Especially since it's only the biological aspect, if she had a child from anyone else you would be fine with the three of you raising it as your own, it seems.
Could it be that you would feel excluded, as a child can only have two biological parents, but that if it's not your husband's either, then you're not the odd one out?
I would definitely appreciate more insight on the moral aspect of it, especially why relationships are fine but not children, and why adoptive children outside of marriage would be fine, but not biological.