The potential for a bad outcome is high.
Son of John,
Re. your post, for brevity I'll refer to your wife's second ex-husband as her ex.
Son of John said:
My wife's ex-husband, her second ex-husband, has spent some time in prison. He will be released soon and she has said that she is sure he would like to "see her". I should explain that she divorced him after he was sent to prison, and that he had no say in the matter.
There's some really serious missing information here. What was your wife's ex found guilty and convicted of, and how long was he in prison?
Son of John said:
I am being realistic in thinking that if he does try to "see her" he would also like to fuck her...
No doubt. You can also realistically assume your wife's ex wants to fuck her regardless of whether he actually comes to see her.
Son of John said:
...if only to bring closure to their relationship.
You must have written this sentence without re-reading it to see how it sounds. When a previously-close but long-separated man and woman meet at long last and fuck, that does not "bring closure" to their separation. It releases their long pent-up passions, renews their relationship and, quite possibly, renews their desires for more... much more.
Son of John said:
I have tried to get her to discuss this, but she is a bit reluctant.
That has the sound of understatement about it...
Son of John said:
I have even said that I would not object, and that I would certainly "understand" why he would feel the need to fuck her now.
Your wife's reluctance is not just a matter of grappling with the question of whether she does or does not want to fuck a sexually-desirable man. (Maybe he isn't... do you even know what he looks like?) In any case, her ex is a man she knew well in a personal sense, because they were married, and he was convicted of something serious enough to result in significant prison time, which implies a felony (at least). If he was actually guilty of whatever the crime was — in our mass-incarceration society, a LOT of inmates did not commit the crimes they're serving time for — his behavior for which he was arrested and convicted may have been characteristic, in some sense, of his relationship with your wife.
For example, if he was convicted of assault, perhaps because he lost his temper in a bar, that may imply your wife's relationship with him involved high stress, tension and fighting. If he was convicted of something like (e.g.) assault with a deadly weapon or armed robbery, that would be even worse. Even if he was convicted of a seemingly-harmless white-collar crime, kiting checks for example, that would imply he's... hm, let's see... how to put this... "honesty challenged." Regardless, your wife obviously saw him being sent to prison as an opportunity to cut him loose. And, she did.
Son of John said:
I have already decided that if he were to show up, I would find some excuse to be gone from the house, leaving them alone.
C'mon, SoJ — you need to decouple your sexual fantasies from the real world.
You're proposing to leave your wife alone with an ex-husband, who has just completed a prison sentence, whom she divorced for reasons that may have included abusive treatment of her or grave mistrust of him because of his personal dishonesty; a man who may be harboring deep anger and resentment of your wife because she divorced him instead of waiting patiently for him to complete his sentence? A man who may be harboring deep-seated anger toward you, as well, for "taking" his wife from him while he was unable to defend himself?
You've got to be joking.
Your most important duty as your wife's cuckold is to protect her, when or if necessary, from ex's and other lovers who become overly-possessive or abusive. What you have in mind has really high potential for a bad outcome. Give some thought to a potential headline in the next morning's newspaper: "SoJ's wife beaten to death by ex-con former husband" ...with a subhead: "SoJ left house when he arrived."
Anyone reading a headline or even a small news item of that nature would interpret "left house" as "fled."
Son of John said:
I know that if I did, they would give-in to their desires, and that he would, at last, enjoy fucking her again.
Maybe so. Or, maybe they would give in to their primal desires and start fighting again with a potentially-disastrous outcome.
Son of John said:
I have also wondered if it would be wise on my part, if he does come, to get him alone for a few seconds and tell him that it is okay with me if he fucks her.
No. It would be much wiser for you to have an in-depth conversation with your wife about this. Ask her specifically if she feels her ex is a danger to her and whether, if he comes to see her, she suspects he might do something violent or otherwise unpleasant. If she continues to be reluctant about seeing her ex and you can't seem to get her to talk about her past relationship with him, you can interpret that as implying it was painful. If she opens up and tells you her ex was verbally and/or physically abusive, you can interpret that as definitely meaning it was painful. In either case, go with your wife's judgement (or apparent judgement).
If your wife doesn't want to see her ex, I strongly suggest not pressuring her. Don't invite him over. If he shows up anyway (perhaps at your wife's invitation or on his own hook), I strongly suggest trying to make sure you're there for the duration of his visit, regardless of the details of what your wife and he do.
—Custer