engaged but confused about naughty thoughts

hey guys i just want some advice or to hear if i am the asshole here. so im 50 and engaged. its been a long time since my last serious relationship before this one, like over 2 decades. my fiancé is in her early 30s. we have great chemistry, love each other deeply, and our sex life is amazing. but lately i keep getting these random thoughts that bug me. when she isnt around or even while were talking on the phone, i get this urge to go look at some stuff online. its not like im actually doing it or fantasizing, just having these weird urges pop up. i feel so guilty and dont know what this means about us or me. am i cheating mentally? is there something wrong with our relationship that i should be addressing instead of hiding from? i dont want to hurt her but also cant shake these thoughts off. any input appreciated.