How are we defining a "crush" here? I define a crush as sexual attraction. You don't stop being sexually attracted to straight men just because you've been with a gay man.
Huh, I'd always thought of it as a mix of emotional and sexual, kind of like the first seeds of a potential relationship. For me personally, if it's purely sexual, I'd use a word like "lusting" or "attracted to." Interesting. Using the word in your sense, I think you're right and that not many would say they stopped crushing on straight men.
EDIT: Interestingly, this how my computer dictionary defines it:
a brief but intense infatuation for someone, especially someone unattainable or inappropriate
where "infatuation" is defined it as
an intense but short-lived passion or admiration for someone or something
With this definition, in particular with the "unattainable or inappropriate" meaning, I'd say straight men are highly eligible for crushing.
I still see it as how I defined it above, however, as I think that's how it's used in most of my social circles.