Husband has been sleeping with the house-help

I am 33 years old and married with two children. We got married when I was about to have the second child. My husband used to live with his sister before that because he didn’t have a job.My heartache is that I recently decided to rummage through his phone — something I had never done before — only to find a love message to his sister’s house-help.He first denied that the message was from him, but came clean after I cornered him. He confessed that he had been sleeping with the house-help for two years, when our first child was a few months old.His reasons for doing so are so petty that I still do not understand why he decided to venture outside the marital bed.I was so angry with him that I beat him thoroughly.