I don't imagine myself having sex with her (not sure if that's what you're saying) but I do imagine kissing her lips/neck, putting my hands up in her hair, etc, things that I could do then and there but would obviously be uncalibrated to actually do. But it very much arouses me during an approach and she gets the vibe too.
Something I like to do is let my eyes drift to her hair or neck and immerse myself in imagining something like that and let my face reflect it, for a second or two, and then look back at her straight in the eyes with a smile as if she caught me doing something I'm not actually sorry about. When my vibe is on point it works a treat.