Was it just the one time? And was the guy a stranger or someone you recognised/know? You don't have to share, but think about the answers. I'd also try and think about what it was you were thinking about of the guy, just the appearance or literally being penetrated by him.

I guess see if it's an isolated event and if so not worry about it, it could have been promoted by an event or something you saw. If it's a regular or more frequent thing then it's important to try and understand it and work it out. I'd also consider talking to your partner, yes it'll be initially hurtful for them to hear but given your relationship they should understand your best intentions and want to help you.