I don't think it's bi-phobic not to want to be married to someone bi. There's a part of this person's sexuality that you will never, ever be able to come close to fulfilling. If someone had a serious part of their sexuality for hair color or body shape, or different fetishes, you can try to fulfill those. But can't be another gender if you are not.

Also, people keep mentioning that it's bi-phobic to be afraid a bi person will cheat. But that's not bi-phobic if you're someone who is worried about cheating in general. You can be worried about your straight spouse cheating. The only difference is with someone straight or gay you feel relaxed around their platonic friends of the gender that's not their preference. If someone doesn't have a gender preference, then there's no relaxation like that.

Also, quite honestly, closeted men from conservative environments who marry young, quite often go outside their marriages to "Experiment" and find out what they are missing. It's not like this is someone who spent his whole teen and college years experimenting and has settled on what he wants. Given they have already had problems with him looking at porn when that's not okay in their marriage, and it's gay porn, I think it's reasonable she's not confident he won't do things against the rules of their marriage to explore that side of himself. Because he's already done that.

I think it's reasonable she is upset that she didn't know about this very important aspect of him before making a lifetime commitment.