I have a sig other, but he's not a GF.
Nonetheless, we've participated in some extracurricular activities when I've watched a couple of really hung handsome muscular (Brazilian) guys take him. He loves to bottom for big uncut cocks.
While the other guy first entered him, I fed my BF my cock. Then, when the fucking got too intense for him to swallow me, I cradled his head in my lap and watched the pleasure spread throughout his body.
That was a few months ago. Every once in a while, when we're fucking now, I remind him of the scene, and he cums almost immediately.
That fact he was so turned on by it -- and continues to be -- turns me on too.