No, his being Bisexual is not the factor that might end his engagement. He's a cheater !!
That he would get married without telling his fiancee about his Sexuality is a huge red flag.
He makes her sound like a broodmare !
Being Bisexual doesn't = Open Marriage. 
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Unless of course he is both bisexual and polygamous
End of conversation
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To omit that would be deceitful.
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