I don't know that Texas, or any other state for that matter, has created a distinction between "sperm donor" biological fathers and the traditional kind. Thus, unless there is a specific statute of which I am unaware, you would have to wait until the baby is born and then petition the court for a step-parent adoption, including termination of parental rights of the donor-father.
Since Texas
bans same-sex marriage, it is doubtful that state would allow a step-parent adoption. If you are not recognized as spouses, you would not, it follows, be a step-parent. So you might have to move to a state which recognizes SSMs in order to adopt the child.
Thus, you had better consult with an attorney to see how you might be able to do this.