Dating 2 guys at once, no commitment declared with either, should I tell them?

I have been dating a guy we'll call J. for a few months now. I tried to get him to be in an exclusive relationship with me, because I like him very much, but he is extremely resistant to the idea of a committed relationship in general because his ex (his former fiance, actually) cheated on him in the past, and he now has difficulty trusting women. I'm also wondering if maybe he's just using it as an excuse because he's not that into me. He definitely has an emotional attachment to me, but it doesn't seem to be strong enough for him to want a declared relationship with me (or he could be hiding his feelings out of fear of being hurt. I can't know for sure). Either way, he has made it very clear that he is not about to commit to me any time soon. So I told him, "You don't want a commitment, I'm going to date other guys." And he responded, very calmly, "That's fine. I can't tell you not to date other guys." But he has also on occasion said stuff like, "I choose not to commit so I can't be upset if the girl ****s another guy." He also didn't want me to meet his friends because, "Last time my fiance met my friends, she ended up ****ing one of them, so now I keep my girls away from my friends." :surprised

So, I'm not counting on anything coming of this, but I still date him (and have sex with him) because he is fun and wicked good in bed. However, recently a guy found me on Plenty of Fish, and he looked cute in his pictures, so I was like, "what the hell, I have no obligation to J" and agreed to go out with him. I went out with this guy, who we'll call M., once, a few days ago. We got along extremely well, he's attractive, he finds me attractive, he's respectful, reliable, etc. And he seems like he is looking to eventually find a committed relationship with somebody. So, M. and I we were having dinner, I had JUST met him, and he took me by surprise with this question: "I actually did some Facebook spying, and I noticed you are friends with J. on Facebook? How do you know him?" I was completely unprepared for this. I was not expecting him to be able to look me up on Facebook because I never gave him my last name, but I guess he found me through other criteria. So, being unprepared, I freaked and said, "Uhhhh... I don't remember." He responded, "You don't remember?" And I said, "Ummmm... it was a while ago... I think it was through a friend of a friend? Yeah, something like that." He seemed to take the hint to leave it alone, and said, "I had football with that kid in high school. Coach used to yell at him when he would bring an umbrella to practice on rainy days." And that seems to be the full extent of them knowing each other. I think if the two were friends, they would 1) have way more Facebook activity on each other's pages. There is next to none. And 2) M. would have had more to say about him.

So, I had to add M. on Facebook when he requested me. But J. called me a couple nights later and asked, "What are you doing tonight?" as if to make plans, but I was in the grocery store and lost service. When I got out and had service again, I saw he had texted me, "Nvm I guess" and I answered "K." Then when I got home, I saw he had posted on my FB wall, "u suck" and not knowing what to say, I wrote, "k." A few hours later, my friend told me I really should delete his comment so that M. would not see it, and I panicked, deleted J.'s comment, and disabled my wall. J. has not contacted me since, but it's not normal for him to contact me every day, anyway, and I realized he can still comment on my statuses, anyhow. I'm just wondering if M. possibly saw that I had a comment from J. which I deleted hours later (I don't think he did, as he was not online in the sidebar, but you never know). And I realize now that the most suspicious thing about my Facebook activity with J. is that I deleted his post, which really didn't imply anything other than that we talk to each other. >.<

So, I already know how it would play out with J. if he found out, because he knew I was dating other guys before and he still saw me and claimed he couldn't object. I'm more worried about what M. will think. J. has made it clear he will not commit, and with M. it is far too soon to even entertain the thought of commitment. I am not doing anything wrong (except maybe being deceptive and not upfront), but I have a feeling that M. is going to ask me about J. again, and I'm just not sure what I should tell him when he does. I do know that if things keep going well with M., I could possibly grow to adore him so much as to want to commit to him, at which point I would stop seeing J. I would never declare a commitment with a guy and still see other guys. I'm only doing it because I only just met M. and the time is not right to commit to either of them.