Why do bisexual people choose to be gay?

As a gay man i have always wondered why a bisexual person would choose to be in gay relationships when they can be straight. I understand if they happen to fall in love with someone but a lot of bi people look for same sex relationships. It would be way easier to just be straight and the person would still be happy about it since they are bi. Is it just like a trend to be gay now days or what?