So far, this sounds like a very awkward approach to polyamory. Your Miss 2nd is married, and cheating? "In sin," as you put it. Right there, she is therefore incapable of poly as it is almost always practiced. Myself, I don't date cheaters.
2nd, did you really mean to say you "like" your SO to death? Odd choice of words.
3rd, no, if you are looking for a friends with benefits, but are telling your SO she can have friends with no benefits, check yourself.
4th as said above, if your established partner needs time to come to terms with this scattershot approach to getting your feet wet in the poly pool it's your responsibility to give her that time, in a loving caring way. Your Miss 2nd needs to respect that. If she can't, she's not the woman for you. If however you throw away your SO for a FWB w some random woman, that's your choice, of course. But you may live to regret choosing a married cheating woman over your actual partner, just b/c you are thinking with your dick. Dicks are great, dicks feel fantastic when aroused, but as mature adults we need to think with both parts, head and genitals. Down, boy.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37