Thank you all for your replies. I have my head on very straight about the whole situation, and like I said, if nothing more ever comes of our relationship, no problem. She is a spectacular woman and Mrs. WS thinks the world of her also (Mrs. WS offered our home to them for the week). I know it will wane, but it sucks right now.
Yes ncmb, you are right, time will do it's deal and take it one way or the other. Either way which is fine with me. And what I've done is put my feelings in a pooka and put the lid on it because to do anything else would be to make her uncomfortable and would dig myself in further. It's nice to know from someone who's been there that I'm on doing what someone else successfully has done.
Forbidden fruit? Not really. More like "elusive fruit".
Mrs. WS knows of our affection for each other and so does K's boyfriend. We have their complete blessing to pursue something more if we wanted to. Just at this time, K doesn't feel she can carry-on two intimately meaningful relationships at one time. And that is fine with me. It's not an easy thing to do.
Would my desire for her wane if I could have her the way I want? I don't know. I guess that is part of accepting poly as a possible way of life. She isn't Mrs. WS and therefore she fulfills a different part of my life. The encompassing idea of polyamory is that no one person can be everything to you, and it's unfair to ask them to be. It's both of you allowing others to fulfill in your partner the things you can't to help them be a more complete person. It's accepting your limitations and allowing your partner to pursue fulfillment in those areas elsewhere instead of having them resent you for not living-up to the "one and only" fantasy.
Which is a liberating concept once it is fully understood. All of a sudden the burden of having to be "everything" to someone is lifted. The pressure is gone and you feel emotionally like someone has lifted a 1,000 lbs off of you.
When our first poly situation came-up a couple of years ago it was with a single guy that had come into our life. At that time we were uncomfortable with the idea of "feelings" developing between swing partners (or at least I was), but it happened. Mrs. WS fell for this guy and this guy developed feelings for her in particular and us in general. He became her boyfriend and one of my best friends. He is also the opposite of me. I'm more the "thinker" type, he is more the "caveman" type. I'm a white-collar management-type, he is an ex-Marine who is in law enforcement to this day. Mrs. WS served her stint in the Army during the same GWI period so they had allot in common there that I won't ever have and I'll never really understand. So he truly fulfilled a part of her I can't and she was incredibly happy. As a side-effect I was surprisingly happy as all of a sudden her being happy lifted that weight of having to fulfill every part of her off of me. That is what turned our minds about poly.
So I'd never want her to be Mrs. WS because I don't want a replacement for Mrs. WS anymore than Mrs. WS wants a boyfriend to replace me. It'd truly "in addition to", not "replacement for". At the root of it all, love is love, but some of the things you appreciate about someone and what they bring to your life is individual to each person.
That all said, I just have to continue being her friend since that is what we are - best friends.
Thanks all for chiming in.