Why not sit him down and simply ask, "is your attraction to her playful? Or do you care for her?"
If he stammers reassure him that it's ok you are attracted to her husband. Then mention that you came across poly.
Explain to him that poly gives you the chance to define yourselves as individuals with in a relationship as having the ability to love more than one. Communication is key to success, remember, and there will have to be guidelines that the two if you should discuss before you jump in the pool.
Fret not, if your communication is as good as you say it is than grabbing the bull by the proverbial horns could be all you need. ( I'm a very blunt person so when I discussed the option with my SO, I got it all out in one breath, terrified he'd look at me with two heads. Instead he said "ok." )
Don't be afraid. Ask us more questions! That's what we're here for!
It's just me, my ramblings and the elevator music in my head.
Me: bi, reformed wild-child turned mom and house maid LOL
Crux: straight/hypersexual, possibly mono?
BC: our son, 5 years
CM: second son 9 months.
Mouse: girlfriend! Status new, feelings not so much.