They are not straight, they are Gay or Bisexual.
Nope.
Heterosexual men do not casually and willingly have sex with other men.
Some men do. I know you are finding it hard to understand, and you are not the only one, that is one of the reasons why they (MSM) went unnoticed for so long. They do not accept the label of gay, because they know "gay" is an identity claim which is not limited to who you have sex with, they also do not accept the label bisexual for the same reason.
Short intro here that gives a few other places to go if you want to understand the matter: https://www.psychologytoday.com/gb/...-men-who-have-sex-with-other-men-in-their-own