Jess, you do not have to meet her husband. You get to determine your boundaries in relationships. But, understand that she may have to introduce you to him if she's to stay in your relationship. This may be one of their "rules." If that is the case, you'll have to decide whether your boundary on this is firm or not.
I've been the single lesbian (I identified as a lesbian for a very long time, and am really just not that into guys, but there are some rare exceptions, so I am more "bi"), and there are some things I will strongly recommend:
Ask these questions NOW, and ask very, very directly:
1. Are ever expected to interact sexually with her husband? While this is often not stated up front, and is often "glossed over," in 80% of the cases I and my other bi/lesbian friends have encountered when dating women in a LTR with a guy, it's something that is going to be expected at some point, even if it's "he just wants to watch." I am NOT cool with that, and it's a deal-breaker for me. Clearly, you'll have your own thoughts on this, and your lady may be in one of the smaller percentage of couples that is actually fine with the woman dating independently.
2. Does the husband has to "approve" of you, and also if he has the power to "veto" your relationship? I highly recommend you do a site search on "veto power," and read up a bit on what that means, especially if your new lady says that yes, the hubby has the power to approve or disapprove your relationship. Veto power may not matter to you, and you may be fine with it (assuming it exists in their relationship, of course--otherwise, it's a moot point), but it's good to know what it can mean in terms of your emotional vulnerability.
3. What level of relationship is your new lady expecting/interested in having with you, and are you both free to develop emotional feelings naturally and organically? Some women are bisexuxal, but not biamorous, meaning the like the girl-on-girl, but don't necessarily fall in love with women. This is important to know going in, especially if you prefer romantic relationships to those that are primarily sexual.
4. Do you have to be "discreet?" Again, this is a deal-breaker for me, but it may not be for you. I don't like feeling like a dirty secret. I don't expect to meet family and friends right away or anything, but I am not okay with never going out to a movie and holding hands, etc. You get to determine your level of comfort with this, of course, but often the married couple's demands for discretion become a problem over time for the metamours. Better to know ahead of time, and adjust expectations accordingly.
5. Will she be sharing the graphic details of your sex life with her husband? If you don't care if she does this, then don't ask. I mention it because, again, I've found that about 80% of the time the female outside the marriage gets used as a "marital aid," kind of like the couple's very own lesbian porn channel. Again, I am not okay with this (if I am not fucking someone, they don't get the details of my sex life); but, for some people, it's either not a big deal or an actual turn-on. So, if you don't care,t here's no need to bring it up. If you do, though, ask now.
I hope this helps. Poly can be a very rewarding life with many chances to meet wonderful people, and hopefully you and your new lady can have fun discovering some of that together!