Is it normal to see girlfriend with another dick?

Is it normal to be young 25 and starting to fantasize about my girlfriend with another guy? I have always been so against any guy even flirting with my girlfriend and over time have eased up but never would consider her getting fucked by a guy. when my girlfriend moved into a basement apartment at her friends house who is a guy I didn’t mind I knew she didn’t like him and I was some what friendly with him but couldn’t help but let it cross my mind after she moved in he stopped in a lot to “fix” things in her apartment and see how things are which I thought was strange. She falls asleep at all hours of the night early some nights normal hours and late it varies but one night I was jacking off while continuing to text her I never really told her when I did unless we were sending pictures but her texts suddenly got short then were cut off completely and it was still pretty early. I was still keeping myself hard as I started to think about it “what if she’s bent over right now getting fucked what if she’s on her back getting eating out” I was mad but picturing all this in my head I started getting harder and thought wtf..I pictured her in like a porno fantasy guy comes in to fix the pipes before you know it she’s on her knees stroking him sucking him off and finishing in her mouth I locked my phone and finished just thinking about it. I was horny when I started I know how she looks sucking dick and she looks good doing it maybe it was a mix of the two but when I start to think about it now I still think it’s a turn on I have a normal size dick, I pleasure her good, we do everything, anal we even lick each other’s asses we have a normal sex life we’ve talked about having a girl join from time to time but never a guy I just don’t know why this is appealing to me all of the sudden. I don’t know what to think and I don’t know if I should tell her has anyone else felt this way?