Nah, it's also something straight people say who are mostly comfortable with homosexuality, while poking fun at uptight people and generally being inclusive and tolerant. A trolling of the ignorant so to speak. People generally like to get a reaction out of people (troll) and fall to peer pressure (being cool). Most likely also a hint of jealousy involved, attempting to conceal this fact by drawing attention to someone else in a humorous fashion, thereby somewhat downplaying their looks, but maintaining your acknowledgment of them.
or something along those lines.
It's not shamed gay people, nor is it only uncomfortable straight people, it also is something people who are mostly comfortable with things say.
so I half agree with DoH and completely disagree with Merkuri (which seems to be a trend in these forums).
Edit: I miss-used the term heteroflexible