You can read a book about how to communicate more effectively with your partner. Another option is to see a counselor for couple counseling or relationship enhancement . The important thing is that you recognize there is a problem, and you're willing to make an effort to fix things. Just like there are many people who are not interested in the same sex even a little, there are many people who aren't interested in the opposite sex even a little. The strong stigma , or negative value judgement, around same-sex attraction and sexual behavior also cuts down on the number of people who identify as bisexual, I think.
Society makes it very easy to be heterosexual and very hard to be homosexual.
It's much easier to admit an attraction to someone whom you can picture yourself having a relationship with, going to the prom with, getting married to, having children with, than it is to admit an attraction to someone who doesn't fit your picture of what your future is going to be like. So I think many people who might have feelings for both sexes unconsiously stick to the easier path, because that's what society condones. Some gay and lesbian people do go through periods of calling themselves bisexual before they eventually come out as fully gay or lesbian. It doesn't mean that everyone who identifies as bi is really going through a phase or hiding their true nature from themselves and others. Unfortunately, you can't always tell.