Is it normal to be turned on by cheating?
I'm a 19 year old heterosexual male, and I have been with my girlfriend for the past five years. Everything has been fine, until one day (and I don't know what galvanised the idea) but for some reason I had a jealous thought of her being with someone else in a sexual relationship. She never has been, but for some reason the thought haunted me for a brief period of time like a plague of paranoia that wouldn't leave. My girlfriend literally doesn't have it in herself to cheat, she's just not like that, but for some reason these dreams came back.
In one dream my girlfriend was having bare vaginal sex with another man and enjoying herself, and I hate to admit it but after thinking about the dream I couldn't help but get hard and have an overwhelming urge to have sex. I am straight and I strictly am attracted to women ONLY, I'm not bisexual and I don't question my sexuality, but for some reason - through my jealousy, my body feels ragingly horny at the thought of watching my beautiful girlfriend having sex with other men for her own enjoyment. I was so concerned about this I tried to look for answers on the Internet. Someone told me apparently it's a natural reaction as your body is preparing to gear up to have sex once the first male has finished with her. But surely this natural instinct should have been extinct from our ancestors long ago?
What is strange is that in reality, I'd be heartbroken and extremely angry, but in a fictional world the thought gives me the urge to masturbate. Surely this can't be normal? We have a healthy sex life and I'm far too embarrassed to mention it to anyone, but what can I do?