I've tried, but after I cum, then I lose the horny interest of doing it and skip it. I think it would be a different story if another guy came over her tits and I would lick it up while fucking her in the missionary position. Or even better if I could suck his cock and have him cum in my mouth while I am fucking her. This gets me hard...

Yes I have because it absolutely drove her nuts.

I have posted this in the past but since it is asked again...

Some of the biggest orgasms my ex would have was when she would lay with her head at a corner at the foot of the bed, I would straddle her face and she would lap at my ass and lick my balls while I stroked and she would be rubbing her pussy or fucking it with a dildo/vibrator and I would cum all over her tits then lick every drop up. She came a few times during it all but when I licked up my cum she would have a HUGE orgasm. When I asked her why she got off so hard over that all she ever said was "Because it's nasty"!