My IC said that men have a primary need to secure a relationship and that they will often times make sure they have one relationship in place before they move to another.
Of course, standard disclaimer this doesn't apply to "all men" but apparently this is less of an affair type issue and more of a male type issue.
As a MH, I can tell you that when I questioned my AP about what he was going to do with his life/wife/future, he immediately replied that he would leave her so we could explore what a future looked like together.
Um, what?? I just want to be your friend and hear you tell me nice things about myself.
Don't internalize this. I'm sure it has nothing to do with who you are or him upgrading and everything to do with knowing someone will be there to keep his bed warm.
Besides...there is no upgrade from a faithful, loving spouse. All his options would be a downgrade.