No STOP sign. BH here.
I don't think the question is whether or not you think of him as third choice but rather whether he feels like the third choice.
I can tell you as a BH that I feel like I am the consolation prize. It didn't work out with OM so I was there to fall back on. After over 3 years of PA my WW broke it off slowly by meeting him less frequently than weekly because she started to see him for being shallow and a loser. Not because of guilt ot conscience or a moral reawakening. She tired of him. She gaslighted, lied, denied, swore before God that she was not and did not commit adultery. We said our marriage vows and I believed them - still do.
I will not date or even look until we are legally divorced, if it comes to that. But I understand what other BSs have posted on SI about wondering if they will be alone forever if they divorce. I wonder that, too.
My WW broke our vows. She rejected me and all of my values and our marriage. We are trying to reconcile but with difficulty. She did not choose me as one IC told me. She rejected me and then used me as a safety net if it didn't work out with OM. I feel like I am a consolation prize to her. In my own mind I know I am better than this. That in all of the important characteristics in life OM does not hold a candle to me.
I am 7 months from DDay 3 and almost 11 from DDay 1. I have called my WW some awful names. Not so much anymore. I still have bouts of anger that come out of no where even when I am having "a good day". (Good days are not so good as they were before.) From what I read here on SI, on Healing Hearts, and in the huge addition to our library on adultery, FOO, abandonment, shame, CSA the anger can continue for a long time. The night before last I sent my WW an email (she was babysitting grand daughters) that ended in F**K YOU. And I meant it. For an hour.
I am closer to D now than anytime since DDay 1. I do not think my WW is fully committed to reconciliation. I think she has a lot of shame and regret but, IMO, only inklings of remorse. She is in IC and is working on her issues. She cannot say "why" - not really "why". She cannot say how she gave herself permission to to, firstly develop an EA with a person she supervised and then, secondly turn it into a PA after a couple of months that lasted over three years. I wish that she could say to me, as others have posted "thank you for giving me the chance to reconcile." She is starting to say "sorry that I have caused you such pain with my selfish actions" and "sorry about what I have done to you by my selfishly actions". Or similar things about specific events.
What can you do that your BH is not 3rd choice. Telling him will not do it. Actions. I can tell you that you, IMO, are very early in long marathon. There are and will be extreme hills and valleys. It will not be smooth and he will not just move on and make it all better. If he does it will not be over. All the stages have to be gone through no matter how long it takes and there will be setbacks and retracing. That is the name of the game he never signed up for. Can you handle it? Can my WW?
BH(me)72(now); XWW 64; M 42 yrsDDay1-01/09/13;DDay2-26/10/13;DDay3-19/12/13;DDay4-21/01/14LTA-09/02-06/06? OM - COW 4 years; "dates" w/3 lovers post engagement;ONS w/stranger post commitment, lies, lies, liesSeparated 23/09/2017; D 16/03/2020