Lily, there are many bi-husbands (and even gay husbands) that are content with only having sex with their wives. Their same-sex attraction is explored and addressed in other non-sexual ways. Just as there are straight men who might fantasize about other women and remail faithful (we do believe those exist, right?). In my opinion, you can use the "lifestyle" argument, when the husband is closeted, but those who choose to openly admit their ssa do not exactly fit the "lifestyle" criteria. Unless by "lifestyle" you mean genuine, close, loving relationship with a person, who happen to be a woman and their wife - then, yes, they are choosing this lifestyle.
In my personal experience, when we started exploring my husband's fantasies, they have been primarily of guys (or me and other guys). At some stage I thought (hoped?) that he might be subconsciously repressing fantasies about other women not to insult me, so I gently encouraged them, but he really wasn't interested. Recently, however, there have been other female characters entering the scene - unprompted. So yeah... bisexuality is complex, nuanced and very different for everybody. I don't know where it will go from here, but even if it was just men he fantasized about - I ask myself - does it really matter if everything else in the relationship checks out?