I think that him wanting her first time is only a factor within the story. Ultimately, we should take that "want" at the same level as wanting a fantasy of any kind (Such as the one being stated at the begining of the story). But if we are to dig deeper into that statement, I am to believe that... since he grew up being a virgin (as stated) until he married, his depicted image of love is somewhat dilluted in fantasy and media. So ultimately, his train of thought is not all that straight.
But still, his point of view isnt all that wrong. If you love someone, you´ll want to know more about them... be it even if it becomes a really selfish and unrealistic wish. As he says, its kind of creepy to want to invade into your others being by exploring bits of their past that dont correspond to you their knowing of (be it her first time).
Finally, I am to say that the story is not ultimately about the factor that he wanted (wished) to obtain that which he should have never had to obtain (the forbiden fruit of his wife´s unknown past). But for that wish to be the calice to turn his actions into the mistake that sets the story in motion. Revealing the real NEEDs he has... (now perhaps by having the objective of fixing his mistakes and straightening things out with everyone... and being honest with himself over how unrealistic his expectations where...).
All I can wrap this up with is... potential... this story has it.
Edit: After reading further down the discussion... I believe many of you are misguided over what the story implies with the MC´s motivation... it is not the factor of him wanting his wife all for himself... he actually never stated that he wanted to undo his wifes past... (the problem lies in) him just wanting to invade into that private part of her life out of mere curiosity... it is curious how far out our minds tend to go when it comes to fantasy and over the what if´s... since one does not actually think of the implications of said fantasies or what they could mean about us.
Now that the MC has the actual ability to do so... his actions will take place, showcasing his true nature... and exposing if what he said was biased or if he actually understood what he was talking about.
Also, the implication of NTR is supposed to be for climax and drama... if it actually happens, it should have plot relevancy, and NOT for the sake of fetish... this is NO hentai