Sex with others is OK if your partner is in agreement with having an open relationship. If she is not then it's not ok.

It's not a matter in your case of whether it is sex addiction or not. Let's back up here. From what I understand you are in a relationship and you have been seeing a couples therapist. If you are comfortable with having sex with  one or more partners per week then you shouldn't be in a relationship with a woman who is not accepting of this. It is not my place to judge you on your sexual behaviour. However, it is important that if this sexual behaviour is what you prefer then you should choose to be in a relationship with someone who is accepting of an open relationship. Clearly, your present partner is not. That is where the problem is. So, you need to ask yourself what is important to you. Having an open sexual relationship and therefore choosing a partner who is accepting of this or staying with your present partner and dealing with the sexual encounters with other women that your partner does not approve of. So it does not seem that you are having as much sex as you can without it hurting your partner. Hey, if it doesn't hurt your partner then go for it. It's your choice.