I think this kind of thing can be an addiction. However when you are with a partner it is a bit of a selfish addiction. (Although I guess most addictions are selfish) I mean, you are just addicted to the buzz, or the ego boost as well as getting a sexual kick out of it.
When I was younger I went through a stage of this and it is surprisingly addictive! That is not an excuse though. You ask "Are you doing something wrong" I would simply say it like this:
If you are doing something that you are unsure/nervous about fully admitting to your partner, then you know you are probably doing something that he/she would not like or be happy with at all. Otherwise, why wouldnt you tell your partner?
So I think you know that you are doing something wrong. Ok you might not be cheating in person but you are still investing emotional and sexual energy into other women as well as your time. I am also sure you are aware that most people would consider this cheating to some degree and most people would be pretty upset if they found out their partner was doing this. How would you feel if you found out your lady was doing exactly the same as you?
As far as the other women not being bothered that you are with someone, well, that doesnt excuse it at all. It is your responsibilty to remain faithful to your partner, not the other girls. They are not in love with your partner, you are. You are the one that will hurt her (I guess) if she found out about this. You have the responsibilty to control your own actions. I have no idea why these girls carry on sexting you when they find out you have a partner. Although I did watch something on TV once that said a man becomes more appealing to other women when he is in a relationship with an attractive women. Almost like it is a competition. To see if they can get your attention away from your sexy or smart partner, which gives them an ego boost. maybe it is that, maybe they just do not care at all and just have their own lust that they want to sate, no matter who gets hurt. It is all pretty selfish. Temptation comes along, you resist it. Thats the unwritten promise you make when you get into a monogamous relationship.
Like Tracey said, I would also be really hurt if I found out my partner was doing this behind by back. First of all I would feel that I wasnt good enough for him or did not turn him on or he was bored with me. I would be hurt that he lied to me and did something behind by back. It would probably ruin our relationship. I would be wondering if he was thinking about them while he was sleeping with me, I would be wondering why he needed this extra stimulation. To be honest the trust would be lost. Considering that is one of the most important things for a healthy happy relationship, losing trust would be a big deal. If I stayed with him I would constantly be wanting to check his phone or emails. If I saw him reading a text and smiling, my first thought would be "who's that" I would be arguing with him if, for some reason he was an hour late home, I would probably go completely off sex with him. I would be thinking "Wow, if he cannot even resist temptation when a girl flirts with him, what would happen if a girl came onto him in person, he would cheat for sure" I would resent him. Eventually we would probably split up, if not immediately.
I would say that you can help it. Saying "I just cannot help it" is a false statement, we all have the ability to resist temptation. You are choosing not to stop because you are addicted to the buzz or the ego boost. In the process of getting this buzz or ego boost you are potentially going to really hurt the one you say you love. I would say if you dont want to hurt your partner, that it is not ok to carry on. Not unless you tell her everything, and both of you are happy with it. (Some people are) I think you should stop, and I think you are capeable. I mean if the doctor told you that if you eat one more bacon sandwich you will die, I am sure you could easily resist that bacon sandwich, no matter how much you love them. We all have the ability to resist. At the moment you do not feel enough risk to stop. Sometimes the risk is even part of the buzz, but seriously it is destructive. Eventually you will have to either find a woman who is happy with this behaviour or risk being in unhealthy relationships filled with suspicion and jealousy if or when your partner finds out.