Is it 'normal' that when you have a fall out with you partner that he seeks out solace with his ex?

I have had a 4 year more on than off relationship with my partner and I acknowledge that we have a very toxic, love/hate relationship. He claims to be a very honest person and I always get the devil in the detail, which often can leave more damage than not saying anything at all.

Prior to meeting him he met his ex through work, they had a 6 month affair as he was married at the time and apparently his ex wife knew of the affair. He chose to end the relationship and settled it seemed for about 7 years with his wife and they had 2 sons. His wife then had an affair which led to the breakdown of his marriage. He then got back in touch with his ex and again had a 6 month relationship with her. Not long after I came into his life.

I have lived with him which proved too much for my mental health due to our difference in concepts of how we felt a relationship should be conducted. I recognised that I was becoming mentally unwell as I was feeling violent towards him and chose to leave and subsequently have bought my own house.

We continued to have a relationship and still do, however living in separate homes is proving difficult and I have every reason to have trust issues with him.

Practically every time that we have had a break up, but got back together, he has contacted his ex and gone to her home. He has told me that on one of the occasions he nearly had sex with her and on another occasion they had sex…he justified his actions in that he said she was stating that they wouldn’t do anything and he wanted to prove to her that he could. Subsequent to the occasion when they had sex she contacted him telling him that she loves him…he informed me that from his point of view she is just a friend.

We have had a recent fall out and again he sought solace in her. He stated she contacted him late at night claiming she was mentally not in a good place and threatening to self harm. As a friend he went to her home and he stated that she was quite intense and coming onto him and obviously had other intentions telling him she loved him several times. He stated that they didn’t have sex and I do believe him, and he wanted to prove to me that he could go to her home and not have sex with her.

Is all this normal? Do other people seek out an ex in times of relationship difficulties? Does this prove that there relationship isn’t just platonic?