Should I feel guilty for thinking about another man?

ok… so I dated this guy for 8 months when I was 17 years old… We stopped dating b/c I found out that he was married (at age 21) and I wanted nothing to do with it! He was the 1st and only person to make me orgasim and now I am turning 22 and I still dream and think about him in a sexual way! I have been with my boyfriend now for 3 years and I love him to death, but I dont dream or think about him in a sexual way… well not the way I dream about my X!! I talk to him once in a blue moon, but just to say hi and catch up (now that he is divorced)!!! I had opportunity’s to sleep with him, but I am not like that, so I didn’t! I just dont know what to do.. is it normal to be dreaming / thinking about another man when you are in a serious relationship?? It makes me feel guilty.. I told my neighbor and she said that if I dream of him a lot and talk to him once in a blue moon then I am cheating in some way.. I was like what the f*ck NOOO… I am just kinda lost and confused on the matter.. does this happen to people often?? Should I sleep with him and see if this weird lust goes away?? The weird thing is I dont like him.. well I like him as a friend, but I wouldn’t consider him as someone I would want with me in a serious relationship of any kind!! What should I do and is this normal??