Perhaps vaginal penetration is where his guilt begins. Talk together about what is going on for him. Consider talking about any issues that may not have been resolved in your previous discussions about this extra-marital incident. It is sometimes the case that unacknowledged power struggles are expressed in the marital bed. Forgiveness is critical to recovering your sexual bond, but it is also important that you both understand what caused the vulnerability for betrayal and why it is you feel that this situation would not happen in the future.
True intimacy requires honest communication.
If your husband continues to experience difficulty with maintaining an erection or your renewed energy for one another begins to feel stale, consider a consult with a competent marriage counselor to ensure you get to the bottom of any implications the affair has for your marriage. However, your husband may need his own counseling to forgive himself. Perhaps his actions have caused him to doubt himself in many areas, and his inability to achieve an erection is his body's way of asking him to take a deeper look at himself. Your husband's betrayal is a deep wound for you because he broke the trust on which your marriage is founded. But his perpetration of disloyalty may have far deeper ramifications for his own character.