And her husband wasn't keeping a secret from her. Unless she directly asked him "are you bisexual?" or "who have you dated before me?" maybe he never thought it was a big deal He could've easily said nothing after that dinner, but instead he told her. Sounds like an honest guy to me.
Your sexuality is a big part of you. If I marry a man and he waits till after to tell me hes also attracted to women id be angry too. Not for the same superficial reasons as her though, but i want to know everything about my partner. Thats not "no big deal". that aside, it also makes it look like he has something to hide
If the husband told his wife before they were married, I bet she would cancel the marriage. She's not mad because he bent the truth, she's just biphobic (like the majority of straight girls). If she wasn't, why does it matter? He didn't cheat on her AND is committed to their marriage. That's what matters, what is she mad about?