There have been many posts regarding the conservative nature of Michael and Vito in the realm of sex. Do we really know this to be true of Michael??? There is really nothing to support this throughout the trilogy (that I recall). With Vito, we sort of get this stance in a "round-about" way. One example is through the way he chastises Santino for his games that he's "playing with that young girl".

No, there is nothing in the Trilogy that literally proves that Michael was straight-laced about sex, apart from the fact that he was checking into hotel rooms with Kay, registering as Mr. and Mrs., and doing the nasty with her before they married. But nothing is depicted anywhere that shows Michael as a philanderer, or a kinky-sex practioner a la Geary. In fact, given his cold and controlling nature, I doubt he had any interest in extramarital sex--or even in marital sex, except to produce more boys for his empire. Of course, that didn't stop him from owning a brothel in Nevada (Tom said "..my brother Fredo operates this place.." which tells me that Michael probably owned it).
The novel at least twice says that Vito was "notoriously straight-laced in matters of sex."

Mike's dismissal of the bimbos in Vegas (when he met with Moe Greene) was an example of his straight laced attitude (and showed that Mike was "all business and no play").

It also provided, for me, at least, one of the funnier moments of the trilogy.... did you ever notice the leacherous look on Al Neri's face as he eyed the bimbos?

A murderer who had morals. It's the juxtaposition of so many mobsters. They are religious(?), are good to their mothers, and are fathers to their children.

I believe that one of the reasons that Michael would not engage in adulterous acts was because he was the kind of man that was always on his game and kept his guard up against those around him. As SC said, "all business and no play." He was never one that would allow himself to be careless and put in a position where someone would have something on him to use against him.

After his passion died with Appolonia, I think that power became Michael's aphrodisiac. Look at his proposal to Kay! It was more like he was proposing a corporate merger than a marriage. Although I think he did care for her (he spoke rather tenderly to her while dancing with Anthony's party), I think he loved her legitimacy, her WASPiness, the assets that he could trot out as it suited his pursuit of absolute power. That was more than enough for him. He didn't need sex, except, as TB said, to produce more heirs.

Yep, actually I watched that little moment about 3 times the other day. Tom has a rather weird look on his face too. Actually, I don't think that scene looked very real on their part. Not their best acting IMO.

Absolutely.

Well put, SB! Too bad we didn't have Sarbanes-Oxley then--he'd have gone to jail.

In addition to what has been pointed out, you can see that Michael is totally uncomfortable at the Superman show; he derides Hagen for having a mistress; and when Moe Green tells him Fredo has been banging cocktail waitresses two at a time, he has no good answer except to say that he leaves in the morning and think about a price. He did nothing to defend Fredo after that comment was made.

As for doing the nasty with Kay, that was in his youth....after he had fought in WWII and it was during his "rebellious phase."

Oh, so I see alot of instances of Michael, simply, turning a blind eye (ie the superman show, Fredo's "banging cocktail waitresses two at a time", etc.). I also see that, given Michael's personality, he wouldn't be that interested in sex, but what I don't recall ever seeing is a reason for us to believe that he holds moral objection to the idea; although I'm quite sure he knows it's wrong (as does everyone).

Ah, I see the young Michael Corleone at Darthmouth...love beads...moccasins...smoking pot...sit-ins...kumbayah...

No, I don't think Michael was of that vintage. He would have been more into racoon coats, swallowing goldfish and That band leader Johnny FOntaine worked for.