You have an awesome wife it sounds like.
As to coming out to your dad... I don't think I'd suggest "coming out as gay" to him... because in your OP, you actually don't describe yourself as gay. You describe yourself as bi with a preference for men (but not an exclusion of women).
both gay first and bisexual second, having realized that the majority of my attraction is to men, but on occasion, there are women that I can be emotionally, physically and sexually attracted to.
If it was me, I'd have a "Kinsey Scale" handy and just explain that I was a 5 (or maybe a 5.5 if the only woman you are attracted to is your wife) on there. And how it applies in this situation.
For reference, if you don't know about it, here's a Kinsey Scale:
I think it would just keep the confusion down as much as possible being as accurate as possible.
Just my 2 cents worth.
ETA: As far as the poly issue, If any family member that you are opening up to is religious, tell them that non-monogamy is Biblically supported. There were many examples of men with multiple wives and/or concubines. If they are not religious, just explain that this is who you are, and your wife accepts you being this way and as long as she is happy, and you are happy, then they should be happy for you.