always remember that in your fantasy, everything goes exactly as you want. in real life though, things can always turn south. what if she likes this big dick too much? what if she starts meeting him outside of your relationship? what if she cuckolds you? what if she starts disrespecting you because she no longer sees you as "man enough"... and the list goes on and on...
how do i know this, well, i've seen this in real time enough times at swingers parties when a new couple shows up and she gets her brains fucked out like never before and the husband/boyfriend starts to realize that it was a big mistake then we never see him again but she will show up by herself here and there.
be careful what you ask for.
if you're not confident enough in yourself to openly talk to her about it then you won't be strong enough to handle it when she becomes the biggest slut you've ever seen to that big dick and never to you. if you're 99% sure she won't go through with it then don't bring it up because even if she ultimately won't do it, she won't ever look at you the same again. hell, it might even end your relationship.
good luck otherwise. my advice based on your few sentences: keep it in fantasyland in your head.