It's normal to feel insecure and while some insecurities are valid, most are the product of paranoia. After being asked numerous times, I decided last month, to answer the question for good.
http://acidlacedkisses.blogspot.sg/2016/02/what-if-he-were-to-cheat-on-you.html?showComment=1456920411241&m=1#c3087470252674175242