Those are definitely good, legit reasons.
First, once you talk about it, you need to decide the ground rules, if any. Do you want sexual non-monogamy? What about emotional non-monogamy? If you only desire sexual non-monogamy, how do you plan on keeping the lines between the two from becoming blurred? If you want total polyamory (sexual and emotional non-monogamy) then you also have to decide if you just want multiple partners, or if you want to use the "primary partner" model, wherein every other lover takes a backseat to your primary partner.
From the sounds of it, you just want sexual non-monogamy. That's what my boyfriend and I do, as well. We have similar reasons that you listed - we're honest, attracted to other guys on a physical level and not very threatened by that... we both realize that just because you're in love doesn't mean you suddenly stop noticing other people. We have some separate interests in bed, separate kind of guys that we like to fuck sometimes, and we're both tops so sometimes it's fun to be able to top the fuck out of another guy. Plus, we're young, so allowing ourselves to still have sexual desires and experiences beyond ourselves seems appropriate and actually more mature than trying to pretend we only want to fuck each other.
For us, it works like this: we can only hook up with other guys once every couple of weeks. When we do hook up, we play it safe: condoms for anal sex. The person we hook up with is not exactly a good friend or something like that. We don't tell each other unless asked, but if asked we have to tell. That rule was awkward at first but gets easier over time - to a point where we'll say to each other, "Oh, you hooked up? How was it?" and indulge in the details. But I have to admit it's still reassuring if no cell phone numbers were exchanged... Let's see, other 'rules'... once you hook up with the guy, you can't hook up with him again (this one might not be for everyone, but it does keep us from falling in love with a fuck buddy). Oh yeah, and you can't "plan" your hook up, ie, "Sorry babe, I can't hang out tonight. I have to go fuck this guy." It should be spontaneous.
That should give you an idea. Our rules are also open to negotiation. They've changed a lot since we opened up. We used to have a no-kissing other guys rule, an all-safe even for BJ's rule, etc. But we're open to discussing changing them too, so we got rid of those for practical reasons.
As for your second question, actually at first my partner hated the idea, which I found very unfair because he knew that when he met me that I was not a monogamous person. We started dating, monogamously, but agreed that we would only be monog in the beginning just to make things less complicated. After a few months, I told him I felt secure and comfortable enough to be open with him, and I was thinking pretty open - fuck buddy open - and eventually he compromised (after thinking about what he wanted, too) and we have the model we have now. He actually uses it more than me, in fact.
It's not perfect. I'm finding it difficult to hook up with people without much interaction first - that was much easier when I was single and it was less obvious that I was merely looking for sex. Also finding the one-fuck-per-person idea a little strange lately. But not sure how comfortable I'd be if he were fucking another guy on the regular.
As for your third question - well, I just brought it up honestly. He knew I was into that from the beginning so it wasn't out of nowhere, although he was uncomfortable at first because he went through the typical "why am I not enough" or "do I not fulfill you completely" kind of questions - which is not the point at all. It took a lot of talking to explain how just because you want to have sex with other people occasionally doesn't mean that the person you're really with somehow isn't "enough" - it's not about having 'enough', it's about expanding experiences. Be prepared for a little uncomfortable conversation, but it sounds like you guys might be okay if you can already at least discuss being attracted to other guys. Just make sure he isn't one of those prissy guys that will "show you the door" if you bring up the subject, one of those guys that will say "you want to have sex with other guys? then you can go ahead and be single!" - although, hopefully you wouldn't want to be dating someone so rash in the first place. But there's a lot of guys like that out there.
I'd also highly recommend the book The Ethical Slut. It has a new-age hippie kind of tone, and is mostly geared towards "polyamory", but it's also useful for people just looking to be non-monogamous in general. There's also plenty of great resources online.
