If parental rights have already been terminated by a father/ex-husband, in the event of a re-marriage, can the second husband adopt the child without the ex-husband's consent?

Provided that the parental rights have already been terminated by court order, then they are just that terminated, over.  The biological father is no longer legally deemed to be the father.  At this point you are the sole and legal parent. As long as you consent to the adoption and your state's adoption requirements are met, your new husband can adopt your child.  In the eyes of the law, there is no other parent from which consent must be obtained.

Since adoption is a tricky process, at this point you should consult with an attorney your area.

Best of luck.