First, Know Thyself. What exactly is it that you crave? If you just need penetration, then yeah - you should totally invest in a strap-on. Easy answer, carry on with being a very happy couple. If what you need is more of a short-term emotional or sexual energy dynamic, then you might consider trying roleplay (probably with the assistance of a strap-on). If what's really going on is that part of you desperately wants to be pregnant, then that's going to have a rather different solution set than if what you really want is to be physically overpowered by something that acts, looks, smells, and/or feels masculine. Of course, it's not just you in the equation, which brings us to our next point.
Second, Communicate. You should be talking about this stuff regardless. Discussing things with your girlfriend can help you clarify what you need, what you want (but can be okay without, possibly indefinitely), what your partner is willing to accept and/or help you with, and what it will cost her. You'd know more than we would about how to bring it up in a way that won't offend or hurt your partner (or at least that will keep the cost of talking about it as low as you reasonably can) but if it were something that you could just discard out of hand, you wouldn't have asked the internet for help. Given that it's something that you can't just drop and ignore, your partner deserves/needs to know ,so that you can figure out an answer that works for both of you, and keeps this whole "very happy couple" thing going strong.
...and a few parting thoughts.
- As Dan Savage notes (if you do decide to go with a threesome) all too often, the couple that walks out of a threesome is not the same as the couple that walks in. If you find this to be a tempting prospect rather than a source for serious concern, then you may not be as happy with this girl as you think you are.
- Expect to make compromises. If you find some setup that works really well for both you and your girlfriend and satisfies everything, then that's fantastic, but don't depend on it. You're into women enough that you're happily dating one... and you're into men enough that you want a man *anyway*. You're already in a place where finding a relationship setup that gets you what you want is going to be a bit tricky - don't sabotage yourself by being unwilling to bend on the less important stuff (whatever that may happen to be for you).
- If you do decide between the two of you that opening up the relationship is a plan, then what you're going into is something called polyamory or poly. I *strongly* recommend that you take some time reading up on the subject before you put it into practice, or even before you finalize the rules under which you'll put it into practice. There are all sorts of interesting and not entirely obvious ways to mess it up, and all sorts of resources out there (online and in book form, free and otherwise) that have useful advice on how to avoid them.