Denying bisexuality falls under the tenant of free speech.
People are prone to denying anything they usually do not like or perceive as not being in their best interest.
The key issue of bisexuality is that for many people it does not functionally exist.
Within your usual m2m context, the fact that this other dude may or may not be interested in women is something that really does not affect me.
Equally so, within his m2f context, his interest in other men is usually kept secret, so for the woman involved, her bf is a perfectly str8 guy.
No doubt, gay dudes play for our team. Str8 dudes do it for theirs. The bi dudes are more frequently switching sides than playing with other bi-folks...