Before you open pandora's box, here's some things to consider that I've seen break many men whom thought it was a cool idea to let their girlfriends/wives fuck other men:
What if he makes her moan like you've never heard before? -- this always happen since the new guy may have a bigger dick, know how to use it well, and have a full arsenal of sexual techniques that will bring out some new sounds outta your woman that you've never heard before. The new guy could also be a fantasy-type come true and she will indulge if you're already all in.
What if he makes her talk like you've never heard before? -- the new guy may bring out the slutty side of your woman that you've never seen or was able to do especially if she picked him out. She may start to say stuff she normally wouldn't, verbally beg to be fucked or humiliated, and whatever more that you've never heard of before.
What if she willingly does what she's never allowed for you? -- the new guy may be able to put his dick in her ass while it was always off limits to you. She will happily gag on his cock but never yours. He can stick his finger in her ass while fucking her pussy but when you tried doing that she yelled at you. She happily titty-fucks his cock but never done it to you. She loves getting her hair pulled but the last time you did it, she told you to stop. The list goes on and on.
What if she likes it too much? -- this is the biggest regret that I've seen guys have. They bring out the sex monster within their girlfriends/wives and then can't put it back away. For most couples, this is a one-way ticket. She may learn that she's a total sex-junkie and will start being promiscuous with or without you whether or not you're aware. I've seen women continue sexual relations with this new guy behind her boyfriend's/husband's back.
There's other scenarios as well but from my experience, these are the biggest and most common ones. Now when I speak with guys who are happily into the cuckold and/or hotwife lifestyle, they absolutely love it and wouldn't have it any other way. Most men are not into this lifestyle at all; many think they do until reality hits. Remember that in fantasyland in your head, you control everything and it always goes the way you want it; in real life though, not so much.
If you're still willing to risk it, then I would suggest by starting to bring up the idea especially when you two are watching porn. Ask her when she's already in the mood how she would feel about another man fucking her instead of you. What if it was one of those big dicks onscreen. Be gentle, respectful, and never push the idea. Let her respond and go from there.
Lastly, you also run the risk of damaging the relationship. You may find out that she's cool watching big-dick porn with you and such but the real idea of it repulses her. She may look down on you wondering how you can possibly want another man fucking her. It can go south pretty quick too hence be careful when you broach the subject.
Good luck. I've seen enough relationships blow up over this, enough guys regret over this, and have been the guy that fucked her in ways her boyfriend/husband was never allowed to or able to.
