I dunno. I've known gay guys who were pretty aggressive about trying to get with girls because they REALLY didn't want anyone to know they were gay. They'd rather be seen as some sex-obsessed neanderthal than gay.
Which is pretty fucked, honestly.
I meant to respond to this after you posted, but sorta forgot about it and got side tracked with work. So apologies for the very late response.
Anyways, I see your point. But from my perspective, Velma was acting aggressive not because she was trying to hide her sexuality, but because Shaggy was so passive in the relationship. He wanted to keep everything a secret from the gang, especially Scooby. He would actively ditch Velma to hang out with Scooby. And even after Scooby found out, he went out of his way to make Scooby comfortable rather than trying to keep the relationship between him and Velma stable. I believe Shaggy did love Velma, but he was too scared to commit to the relationship at the time. And by the time he was ready, Velma just wanted to be friends.
As for the producer's claim that Velma was written to be gay, and that everyone but a few people saw it, I find that to be bunk. I remember being part of the Scooby Reddit back then and the topic of Velma and Marcie being a couple had been brought up a few times while the show was running, and most people shot it down. They came off more as old friends who had drifted far far apart finally reconnecting with each other before the universal reset, than two people who had romantic interests. I view this being no different than what JK Rowling is trying to do with Harry Potter.
Of course it's always possible for two old friends reconnecting could start developing romantic feelings for one another later on, but that doesn't mean a relationship will happen.