Wow - thank you all for the replies... There's a lot of good things to think about in there, and I certainly think, some very solid advice.
RE: "open vs poly": Yeah, this is primarily an Open Relationship/swinging, I guess, if we're getting really specific with labels. She and I are primary, but we're open to secondary lovers and affairs, even some lighter emotional involvement, but we're not full poly.
RE: STDs - HPV can be spread easily even when wearing a condom - it affects the entire genital region and spreads through skin-to-skin contact. Condoms only cover the penis, not the surrounding area. HPV has even been known to spread through kissing and handshakes. It can also be vaccinated for now (some strains), and we actually HAD HPV almost 10 years ago, but have shown zero signs of it still being in our bodies for the past 5 years, so, yeah, we know HPV sucks... but it's a risk you take even if you DO wear a condom. She assures me that they had a talk about STDs, that he had a test done less than 2 months ago, that he's free and clear, and hasn't been with anyone since the test. Well, other than her.
RE: Breaking Trust: I'm not particularly worried about STDs in this instance for the reasons I gave above, but yeah, it DOES worry me that she broke her own rules - our trust, and she did so without hardly thinking about it. This was tough, and we talked through it - but it was the first of a number of things that I didn't like about everything; as I said before, they kinda kept coming one after the other. Neither of us likes condoms at all, so its difficult for me to blame her for not wanting to use them, because I don't like to either. Maybe she felt like she could break that rule at will since it was she who was so insistent upon it in the first place. She later told me she was mostly afraid of me getting someone else pregnant.
RE: Drugs - She told him she doesn't want him doing coke around her at all, or before he's going to be with her, and she says he agreed to that... In fact, she said that he has stopped doing it altogether. Whether that's true or not, I don't know. The Steroid thing, she still has not pressed him about... I suspect it's because she knows that the roids are a major part of the reason she's having so much fun, although she was rather surprised by how small his testicles are compared to mine. In any case, I certainly don't want him around our son, and don't expect that he will ever be. Were he to make an unexpected visit to our home in an aggressive and drugged-up state, he would very rapidly discover the fact that I am more than capable of defending my family, and will not hesitate to do so.
Very interesting notion that it may be easier for her to be treated that way by someone she doesn't care about... I never looked at it that way. In many ways, it makes sense - and I've honestly always been uncomfortable being particularly rough with her and other women simply because I'm afraid I'll hurt someone. I don't like hurting people, and have never taken any pleasure in it, even when it was necessary to force submission out of someone who was armed and dangerous during a physical confrontation. When my wife and I are rough in bed, I have never felt 100% comfortable with it, though I like how some of it turns her on - that's what gets me going, her being turned on, and that's one of the reasons I'm fine with her enjoying another man - the fact that it turns her on turns me on, and it snowballs. (in the metaphoric sense, obviously)
Again, thanks so much for the replies. We're going to talk again tonight.... the rule-breaking/trust-betrayal is going to be a topic of conversation. I think that's part of why I felt emasculated - as though this guy must be superman to make her throw the book out the window so quickly.