Did he see anyone before meeting Mama Corleone?

In the novel it says Vito was very moralistic about sexual matters, so it is not likely he did.

Given Sonny's reputation as a womanizer, it must be somewhere in the family.

Remember in GF2...he says to Genco about his girlfriend...."She is beautiful.....to you she is beautiful, for me there is only my wife and kids." He was a moral man when it came to his wife, he did not allow her to get mixed up into his illegal business, so she would not get in trouble, he was more concerned about his family and their survival, rather than cheating on his wife or hanging with other girls.
He also scolds Sonny for fooling around after the Sollozzo meeting.
You notice Mike never strayed from Kay after they were married.......I think that is more from his pop then Sonny's behavior.

But even he couldn't resist a thunderbolt in Sicily

Yes but Michael and Kay weren't married yet, so "technically" he didn't cheat on either of his wives

Michael didn't know how long he would be in Sicily....he didn't even know if he would be able to ever go back. Kay was just his girlfriend.....he broke up with her before he joined Sonny and the family.....he didn't want her to be involved.
I am sure he didn't want to live alone in Sicily........but he never cheated.

Well, Vito came to America when he was twelve. He marries a 16 year old girl eventually, right? I'm not sure how old he was when that happened. Certainly, he dated other girls/women before he married. His road to the becoming the Godfather apparently started about the time he was in his early twenties. So, he had plenty of opportunity to date, I guess.

BTW, don't you think the GF (both the book and the movies, possibly but GF3 that is a little more direct IMO) is quite reserved (if not puritan tout court ) when it comes to love and, most of all, sex? My opinion is it would have been better to deal a bit more with these topics: it would have helped us understand everyone's psychology better. All in all, we only know Sonny was quite sexually exuberant, but I got this feeling Sonny's character is more a caricature when it comes to that peculiar facet.

What is everyone's thought?

don't forget he was a Sicilian, if you know what I mean...

I think the novel is full of un-necessary sex. The movies are better. IMO more sex scenes would have spoilt the movies. About love, I am not so sure if more scenes were needed.

SVSG, I'm not talking of detailed sex scenes, of course. You can suggest without actually showing. And BTW, that's far more intriguing. A little less blood & shootings and a little more hints about the intimate life of the characters. That's what I would have appreciated.

RVA: Was he any good with women? He had 4 kids with the matronly and morbidly obese Mama Calzone thats more of a chore than most men could handle.Harpoon her a couple times and Clemenza starts to look good.

RE: Michael and his marriages. Not only did he fall instantly in love with Appollonia, but he also felt that his relationship with Kay was over. He assumed that once he had killed Sollozzo and McCluskey, she would want nothing to do with him. In the book, she pursues him, not the other way around.

People were more reserved in the 70's and that idea would not fly with the studio. I also think it would undermine the movie if they included the deleted Mike / Kay hotel room fooling around scene.

wasn't in late 60's the sexual revolution began?

On the contrary, I believe it would have added warmth and humanity to Michael's character, especially in contrast with the cold-hearted Don he was to become later.

wow, what a refined thought!