If you want this to happen, refocus her fantasy on the aftercare part of this fantasy. Let her know how turned on you get imagining her laying there afterwards, sweaty, mussed hair and a gaping pussy filled with her lover's cum. Tell her how you've never loved her more than this moment, seeing her after such a display of sexual bliss and pure , wanton ecstacy. Have her imagine you kissing her lips that were sucking on her lover's cock, playing your tongue over her breasts and working your way down to her pussy that was ravaged by his girth. She can think about the contrast of your soothing tongue on her pussy lips to the obsessive pounding she just received. Then imagine you slipping your cock into her velvet folds and making sweet, gentle love to her in your effort to reclaim her.
In other words, bring the aftercare portion into it and see how she responds to that. Plant that seed.
If you don't want this to happen, carry on my friend.