I was so thrilled to discover, my senior year in high school, that girls were even an OPTION.
That's not really late in life, but I had been boy-crazy since about 7 years of age. It was a bit of a surprise.
For me, dating someone new always turns me into an awkward teenager again. That's part of why it's fun.
I 'passed' in the lesbian community for about 10 years (and two girlfriends). I didn't lie if it came up that I was bisexual, but I never brought it up on purpose. I believe there is still some stigma; a lot of lesbians don't want to be bothered. Obviously, that's a different issue for your boyfriend's wife. The woman who was my 'first' was only hot for me because she would be my first. Actually, it was the only one night stand I ever had that is a pleasant enough memory for me. She chased me for weeks. The thing that made it the most different from one night stands with boys was that she called me the next day, to ask how I was doing. I found that to be an incredibly sweet gesture...I felt very cared for, even though we were both very clear it was nothing more than what it was.
I understand that knowing others poly experiences helped you; I would just offer a caution. Sometimes people tell horror stories. Those aren't very helpful to what you need to know. I have, more than once, allowed others stories to cloud my judgment of what I knew was true. Worked up all kinds of fear in myself, even though I had no rationale. And of course, what I knew was true was how it turned out, and I had wasted all that adrenaline for nothing.
I would encourage you to not get to hung up on whether or not there's sex going to happen. See if she has any attraction to you, find out, take plenty of time to find out, if you two have anything to build a relationship on. Then worry about things like first and how.