I agree with you. It’s completely understandable that he may not have been a fan of making Jon bisexual. There are so many ways to reword some thing. He could’ve easily have said, “I think that’s cool that Jon is bisexual in the new comics. I think that’s great. More representation. But as far as I know my character on the show is straight. But if they were to go down that path and make John bisexual, I’m all for it.” See? Simple. I believe his costar Alex even said that he would be down for playing Jordan as bisexual too. I think he said, “I’d be honored“. Not sure though. I think it was an insta-story question. Seems more supportive than his on screen brother.
To kind of add to what you are saying, I wanted to point out this interview:
https://www.cbr.com/titans-tim-drake-jay-lycurgo-excited-to-explore-bixsexuality/
Titans actor Jay Lycurgo was put in a really similar situation to Elass last year: The character he plays on
Titans, is Tim Drake, who in the comics has been Robin and Red Robin. In the show, he doesn't have a super-identity yet. Tim had only been portrayed as being interested in women from his debut in his first appearance in 1989 until April of last year, which is a much longer history then Jon Kent has. Tim started dating a boy in last year, after
TItans filmed, and Lycurgo would have been taken a bit by surprise just like Elass and Garfin would have been. But Elass and Garfin reacted very dissmissively, clearly not wanting to add that to a storyline, in a way that I won't label as homophobic exactly but was definitely disappointing, while Lycurgo's answer was much more positive.