Please examine your biphobia
I don't need to 'examine' my feelings about gay or bi relationships, I already know my feelings about them. Let me me spell them out to you just for the record Loki. I have many gay friends that are literally the light of my life, I am proud of them - happy for them and delighted that in many cases they have found life partners.
What I am not okay with, on any level is deceit.
If someone is bi/gay and entering a legal and life long commitment with someone else, then it is essential in my view that they are both completely honest about it. They could get married anyway, but the other person has made an informed choice.
I would have a problem with the dishonesty of keeping this kind of secret. Being bi/gay does not make you any more or less likely to have an affair, but it does raise the prospect of a lack of fulfilment in the bedroom, and that would need a great deal of thought and discussion on both sides before marriage takes place.
Any form of dishonesty kills trust
Long lasting marriages and relationships are built on trust
It is not about the sexuality, but about honesty for me.