Is it normal for a female to stay friends with a male friend that went to jail fresh out of high school

First I would like to start identifying how positive it is to be in a place to identify that a situation is triggering negativity in your current relationship, and working to look at addressing the situation. When we have been doing something for a significant period of time, 20 years at that, it can become hard for us to be open to looking at the situation through another person's lens. There would be some key factors that were we working together I would want to clarify, such as when you met your now husband, was this person someone that he knew about, or has this been something that he has been made aware of further down the road into the relationship. If he did know already, was it something that he was ok with initially and just now started to have a problem with, or has it always been something that he was not really a fan of. And also, how long was the initial relationship with the man who is in jail, as that would play a part in how significant this person's role in your life might be. 

When we are in a relationship, there naturally has to be some boundaries and a healthy give and take on what other parties are allowed to be involved in, or taking time from the relationship, and those boundaries are something that have to be mutually discussed and agreed upon by both parties. If your husband is uncomfortable with you communicating with this person, is he making sure that he himself is not communicating or interacting with anyone in a similar fashion that might make you uncomfortable.  Creating healthy boundaries is a part of relationships that can be hard when the two parties involved have different ideas of what is appropriate or acceptable in the relationship. I would imagine in this situation, you know first hand what your intentions are with this other person, so you know that you are (just a generalization based on the limited information) being a supportive person to someone that you knew in the past and that it is not meant to be romantic or anything that takes away from the marriage and life that you have created with your husband. However, from his point of view, he has no idea the full basis of this friendship/former significant other relationship, and so he is only able to see that you as his wife are giving time and energy to another man. For the male ego, and even the female ego if the roles were reversed this is a hard thing to be accepting of. Since this is a person that you were in a romantic relationship with, that then did something significant enough to be in jail 20 years later, there is a naturally assumed level of care that is had for this person that can be very upsetting to the other person in the relationship. Have you taken the time to sit down on your own and really process through what about this person leaves you feeling that 20 years later you are still in a place to care enough that you communicate with someone from that long ago, after marrying another man and having children. Taking the time to process our own intention in a situation like this can really be impactful. Have we been doing it for so long it is a habit and we feel obligated to continue, did it start off because we had the romantic feelings and it has become a platonic friendship over the years, do we feel a link to our past by keeping in touch with this person, does this person remind us of a time of our youth that we have fond memories of. There are so many reasons why we continue to keep in touch, and while the majority of them are completely innocent, again, your husband has no way of knowing those intentions unless you communicate them with him and it can be really easy to create an idea in our minds, when we don't have the proper facts, that it is something bigger than it actually is. 

Therapeutically I would encourage first processing within yourself as to why this communication continues to be something that is so significant to you, even though it clearly is bothersome to the man that you have chosen to spend your life with. Then having an honest conversation with your husband on those personal boundaries and working to create a sense of what either of you are comfortable with and what you are not ok with in the sense of your relationship. This is something that you guys can ideally come to some healthy realization to and be in a place to feel better about. It will take some give and take on both sides but is worth it for the health of the overall relationship.