Hi I am pretty young. So what I am going to say , like I understand if you won't take me seriously. But , it's okay. It's alright , feelings don't work that way YK. It just happens , maybe you find him handsome ? Maybe you liked something about him ? It's okay. It's alright.
Yea he's married , as long as you're not acting upon it , it's okay. I understand that you're feeling guilty etc but it's okay. And yea it can be painful and unhealthy especially if you want to actually get into a relationship with him but I think reminding yourself he's married and making a list why it wouldn't work out between you both ( compatability etc ) might help. Take care