You'd be shocked to learn that plenty of 
gay dudes fuck women and still call themselves 'gay'. 

! I know! Shocking! But still kinda funny.
 Tons on here too. Look around. That's why I don't care much for what ever labels people give themselves. Really doesn't matter.
		
 
		
	 
Isn't that a bit of a bizarre notion? 
I would say that for people who engage sexually and/or romantically with both genders, but choose one particular label that doesn't adequately fit their behaviour, might do so out of ignorance (assuming that bisexual people must be equally attracted to men and women at all times, and in every sense), or else fear of rejection. As we have already seen, many people are extremely adverse to bisexuality and individuals who are primarily homosocial or heterosocial might feel compelled to define themselves as gay or straight, in order to avoid tensions within their social circle or becoming the object of attacks. In short, constructing an identity takes a lot of time and a heavy emotional investment, and many bisexual people might feel the need to remain "closeted" because they fear becoming estranged from the friends and loved ones who could feel deceived or lied to if someone they are close to admitted to doing things that deviate from the norm.  
I usually lurk in these forums and I have rarely, if ever, seen anyone express that they are gay but feel attracted to women in any way. I actually find it bizarre that in a place like this, where people enjoy relative anonymity and have lots and lots of information regarding what constitutes a particular sexual orientation, some would feel compelled to engage in semantic games in order to hide their true orientation. Aren't we a little too far down the line of understanding sexuality to be playing the "I do this, but I refuse to be described as someone who does it" game? Yes, some bisexual people are viewed with mistrust and even demonized, but it's precisely because of their linguistic muddling that many refuse to at least make the effort to try to comprehend who they are and how they experience things. In short, visibility is they key for acceptance, and if they hide, most others are not going to be willing to even tolerate them. Sadly, it takes courage to convince people of your normalcy (don't we gay people know?).
So, to make matters clear, it is possible to predominantly experience same sex desire both romantically and sexually (be homosocial, in short), but be just as bisexual as any other openly bisexual person. That's why it's so odd to find people who engage in this "I am straight/gay, but sleep/seek relationships with the same/opposite gender". In fact, I understand people who reject these individuals, up to a certain extent. If you cannot be honest about this when you are already exposing yourself to condemnation because of what you 
clearly do, how can anyone trust you?