See, that is why I said "we seem to be talking past each other". Apparently both of us agree on all the stuff - but I feel (and certainly you too) that the essence of what I am trying to say is going into the void. So I am introducing this to explain why I don't see a difference between heterosexual or homosexual on the one side, bisexual on the other. Because I have not seen (or understood) any difference you pointed out except for the defining part "like both genders" and I don't see how "potentially likes partners other than their spouse of a different gender than their spouse" (italics are the difference to homo or hetero people) makes a difference here.
There is no difference.
Bi, homo, hetero. It doesn't matter a jot.
The person in the relationship will still find other members of their preferred sex, or sexes, attractive.
It only matters if they act upon that.
I already agreed with your point, that bi people are unquestionably going to be exposed to more opportunity for infidelity, as they have a pool twice the size.
The sentences of your OP I did not understand was
Antonio is bi. He will always be bi. He will also always be someone who cheated/cheats on his wife.
Hence there is no fixing the 2 issues at the root of the problems with their relationship.
Hence their relationship cannot be fixed.
If I slightly adjusted this to
what would be the difference? You say
The obvious difference is that he's heterosexual in your statement.
The end result is no different.
He's still attracted to other people and he's still unfaithful.
Regardless of his sexuality.
which to me seems to imply than monogamous relationships with non heterosexuals (or I guess non homosexuals) will not work, and I still fail to see why.
I don't understand how what I said causes you to infer that?
A monogamous relationship can work for a bi person, of course it can!
Him being bisexual simply means that he will always find both sexes attractive.
Him being unfaithful means he's acted on those attractions already.
An unfaithful relationship, with any person is not getting fixed.
You can patch it up and continue, but it's never going to be a true trusting, loving relationship.