If I had an affair with a woman going through divorce, am I liable in any way?

Texas is a no-fault divorce state, which means that the parties do not have to list a reason for the divorce only the generic reason that "the marriage has become insupportable."  Some states, which are fault divorce states, do require the parties to allege or prove a reason for the granting of the divorce or for certain divisions of property.  Which that said, there should(emphasis on should) be no reason for you to be drug into the divorce case since fault is not an issue.  You cannot be drug into the divorce as a third party-- so if you are concerned about personal liability for causing the divorce, you are off the hook in that respect.  However, you may be drug into the divorce in other ways.  If one spouse is alleging that the affair somehow impacted the marital estate, the emotional or financial stability of the other spouse, or emotional trauma to the children, then they may want to prove that the affair happened as part of their argument for certain custody arrangements, a greater share of the marital estate, or marital support.  You could be called to court to testify regarding the details of the affair.  Unfortunately, despite Texas's "no-fault" approach to divorces, many do get ugly and matters like these are frequently aired in public courtrooms.