Can someone help me before i explode!!?

Ok... so lately i've been having these disturbing thoughts about my boyfriend of one year and its really eating me alive that i find myself having a hard time sleeping at night. my boyfriend is very sweet and the love of my life hes the guy i want to spent my life with forever. We have our ups and downs lie any other decent couple in this world but 5 months later in our relationship i've felt less attractive or uglier to him. i never get called pretty or beautiful Unless i bring to his attention how im feeling hideous and like he'll say ''Noo your beautilful baby''. We'll we are teenagers and im his first girlfriend. but my main worry is do guys or all guys have erections over other girls? And does that me im not pretty enough for him or im unattractive now? Like he said at the beginning of our relationship that im the on girl who turns him on? could that even be possible just ''1''girl making you aroused? Ever since then he never brought that up again which makes me wonder... But yeah i once confronted him about this and he goes off saying '' no im not that type of guy'' hm.. So lets see we were at school hanging around and there was this girl ( personally think shes prettier than me) walking by and i saw his eyes follow her around the corner. Then lets say ( gives her a name) sarah walks by 3 days later and i notice his hands going in his pockets? sigh* maybe he got erection..:( from her. well guys /girls do they lie about that and could my bf have been lieing to me so i wont get upset? I mean is it fair that i'm faithful trustworthy and id never do that to him with another guy... that mentally cheating right?