I think it's harder for bisexual men to be in straight relationships because for some reason it's harder to find girls who are okay with the idea of it. Society puts all kinds of bad associations with bisexuality in general, but a lot of women find it creepy or weird to be dating a bisexual man unless they themselves are bisexual or accepting of the LGBT community. This also has a lot to due with gender roles that society has ingrained in us. I think a lot of women feel like if a man likes other men, it makes them less 'manly' and find that very hard to deal with.
I personally am bisexual and in a straight relationship, and don't find it at all difficult. I was honest from the start about my sexuality and he's completely okay with it. However, I've never had a relationship with a girl, and that doesn't make me any less bisexual than someone who has. Just because I haven't had experience with a girl, doesn't mean it's difficult for me to have a straight relationship. It's not something I want to pursue currently because I'm happy with what I have.
I think it all has to do with honesty at the start and the willingness of the partner to accept it and not judge the other person on one aspect of their being. It's hard for people to understand that bisexual does not mean poly-amorous or promiscuity at all.