It's the expectations people have, their conditioning from society, and inability to overcome fear.
What I've seen is guys who are conservative, right wing, republicans, law students, frat guys, athletes, and/or guys blessed with amazing bodies they tend to stay with women because they cannot let themselves be fully open with another man either in public or private.
Then there is this: 1 in ever 10 men who identify as straight, who are single, have a girlfriend, or are married to a woman, have admitted to having sex with a man in the last year. Why? Simply put, there are sexual needs that the woman cannot meet, such as anal sex, and the man refuses to tell the woman about this need, causing the down-low lifestyle.
It's the lack of education really, as well as generalization and classification. There is Gay, Straight, Lesbian, Bi-Sexual, Pan-Sexual, Ambi-Sexual, Omni-Sexual, Transgender, etc. What everyone has to do is get rid of the label and just admit what they specifically want from who.