why does he have to text other girls!!? 😞😞

So me and my fiance have been dating for 5 years now, and ever since we began dating he has constantly been finding girls to text... one dies off and another one shows up, some are mutual friendships... others are troublesome. Our relationship has been alot of distrust on my part because of this, its never been as bad as it is not with what the texting involves.. no nudes.. or any hookups, all purely over the phone via text or social media. I have cheated once, and hes never cheated. To my knowledge im pleasing him sexually, and ive asked him several times if theres a reason why hes looking for attension elsewhere. Sometimes i think its because hes young (21) and hes going through a phaze, other times i think other men his age know better. Ive confronted him reguarding it.and he says thayre just friends and that im tripping and have to be more self confident, that if he wanted to leave he would have... what should i think ladies?