Old flame pop up out of nowhere, after nearly 10 years.?

I was in love with a guy , i met right after i relocated in the downtown area almost 10 years ago. but this guy unfortunately did not feel the same about me, after a while of pursuing him i gave up, one day i went by his home, one of the guys that lived in his building started telling me negative things about the guy i was in love with .that he does'nt love me etc, but i had already knew that , but i was so infatuated with him. I don't know i guess i just love bad boys you know. but any way i pretented to go along with what this other guy was telling me, so we started dating, but i only dated him to make the other guy jealous, after a couple of days a big fight broke out between them, the second guy whom i really did'nt love ask me to choose, but i chose the guy i always liked .that was the last time i saw him, until a little over a week ago. but since then i've become a christian and part of my belief is to avoid adultery & fornication, tell me how is it that i cannot stop thinking about this guy, i mean he was'nt even the one i was even in love with. he still looks good still, and then too i have put on a lot of weight since then, please help.