Heteronormative, yes. Cis less so, I'd say.

Anyway, considering the time it first aired, it's almost revolutionary.

But that's a running theme with the old Trek shows. I'm currently on a rewatch and a lot of episodes are still very relevant today. Yet sometimes there are moments where I think ”Yeah, it's fine, could have gone a step further though“ but then remember that stuff's partially as old or even older than I am, and put into context of it's time, it's pretty neat again.

I know what you mean. The implication in Jadzia's statement is that she's typically adhering to straight relationships depending on her gender identity at the time. That being said, if you watch the show and are willing to be a little generous, you can make the claim that she's pansexual. The actress explicitly has said she wanted to portray her character as pansexual. Also, the trill retain the memories and wisdom of previous partners. It's hard to imagine that Jadzia wouldn't feel attraction to her previous partners. In fact, if you watch DS9 S04E05 - Rejoined you'll see Jadzia kiss her former wife and claim she still feels love for her. In addition, that kiss was one of the first televised lesbian kisses!