Trust or Not

I need a little insight from all the ladies and gentleman out there with a bit of common sense and wisdom. Here's the situation.

Mr. X and I exchanged telephone numbers in January of this year, began talking in February and finally went on our first date in March. We've been dating exclusively since March and act as bf and gf even though we haven't verbally said so. I'm not rushing into anything so I'm not interested in firming anything up right now. However, we are very much into one another and could see ourselves marrying one another. We've had many hard conversations already about sex, honesty, AIDS, FICO scores, children, geographic location, religion, etc. We're so into each other that we have no problem laying all this out in order to build a solid foundation.

Mr. X has that look. You know.. That look that a lot of women go for. He's the type of guy you'll pick out of a crowd, say "DAMN!" and do your best to flirt with and slide your number to. With that said, Mr. X has had quite a few sexual partners in his past (like many guys). In some of our discussions, it has come out that Mr. X used to talk to a few women at a time...have a woman come over, have sex with her, and then want her to leave, etc. He basically told me that he was only interested in one thing, mainly because none of these women have been "the one". So in essence, he was a player, a dog, disrespectful to women (perhaps they were disrespecting themselves too), etc. You know.. the normal way men are when they're out there sowing their wild oats and being young and experimental. At least that's how I see it. He doesn't think he was a dog. He thinks he was just young, doing what a lot of young men do. I hear him on that, but I'ma still call him a dog. We'll just have to agree to disagree on that.

So here's the issue. Of course he's saying all the right things - he's not interested in talking to anyone else, I have all his attention, there's no one else out there for him, etc. He's sincere. That much I can tell. I'm not stupid so I can definitely vouch for him. There are clear, solid actions behind those words. He's already called past females and told them not to call him, he's given me the keys to his place, he's met my mom and dad, he chooses to spend his every waking minute with me... But I have to question something. How can a guy sleep with a whole bunch of girls within such close proximity to us getting together and then all of a sudden turn around and not do it anymore - like a complete on and off switch. (Remember, we began speaking in February, which is also the same month in which he had his most recent sexual encounter.) He said something about my affect on him, which I buy. I mean I know the stories of dogs who one day meet the woman of their dreams and don't want to be dogs anymore. But is that truly possible? This turning over a new leaf? Now... all of a sudden... at the drop of a hat, you're no longer this dog that you used to be? I'm a female so I don't know but don't guys have to work themselves out of that? Doesn't it take time? Or does it truly just stop with the right person? Then I start asking myself why should I be special? Why should I be the one he respects all of a sudden? But then that means that I'm discounting my worth which I don't want to do.

So what do you guys think? Is there a transition period that men have to go through or can they just turn it off? Can I trust right off the bat considering his past? Are past's truly important?

Male opinions will be extremely valuable.

Thanks in advance.

Ms. Trust-or-Not