Men are more bisexual than women
Men are far more sexual than women. Men can produce millions of sex cells at at time, their sex drives are constant, and their ability to reproduce continues until death. In contrast, women can only release one egg at a time, their sex drives are relatively low and non-constant, and their fertile cycles end around the age of 50.
Because men are more sexual, they are also more bisexual than women. There are tons of straight-identifying men out there who can swing to men if women are not present. It's an organic need for men.
The idea that women are more bisexual than men is a falsehood that's been promoted by the porn industry. As women are rewarded for pleasing men sexually, they will often say and do anything that causes a man to reward them. This includes falsely claiming to be bisexual.