Webcam

I have been in a relationship with my partner for a year and moved in with him 3 months ago. I have just discovered that he is a member on a swinging website and that he masturbates online over a webcam with other women. I confronted him and he says it is just curiosity and he will stop doing it as I told him it upsets me as I see it as cheating. he says he has never met up with any of these people in person. need some advice, please.