The most sexiest thing is to see a woman with cum all over her face.

My suggestion, tell your wife that it is a reall turn on for you, do not be too pushy, but bring it up occasionally.
That's how my wife and I got into swinging. Little by little and until finally we both to the step over that line.

Of course, that same way that she let's you do that, try and find out what pleases her that she won't talk about, and explore that with her.

Dude.