The best safe-guard against getting pregnant from another man is if the wife has had her tubes tied, or if the other guy has been "fixed".
The friend who fucked my wife the most times had had a vasectomy, so he was completely safe. There was no problem with him cumming in her pussy, and all three of us thoroughly enjoyed the sessions in which he kept on slamming it to her as they both had screaming orgasms.
I was, of course, masturbating as I watched, and when he pulled out, if I had not already cum, I would shove my cock into her open cunt, still full of his cum, and have the best cum imaginable.
Fucking my wife when she's full of another guy's cum! Nothing better!