If you have concerns in whatever form.....don't do it...simple as !!
If you are so concerned that your wifes head will be turned by someones appendage, as that is the only thing you think pleases her, then I would call in to question why you are swinging?
So yes, best you leave it alone.
A fucks a fuck basically, any relationship is surely built on more than that.
If you think thats gonna make or break your relationship I'd be questioning:
is your relationship what you thought it was and why are you doing this?
And most importantly I'd be speaking to your partner about it.
Just my opinion, but I figured you wanted it, thats why you asked in the forum.
Hope turns out well :-)
I think it has all been said, is it your fantasy or hers would be interesting to know? I say this as I do get off on the wife having a large cock inside her, but many of her favourite meets have been with guys with much smaller ones. far more to good sex than just the size