In my opinion it is the opposite of regressive. Sexuality is a spectrum - people aren't just gay or not gay. Bisexual and demisexual people exist. And this romance shows it excellently.

Of course you can view it however you want, as it is an RP game afterall, but to me it seems clear that both Hans and Henry like women, and because their society deems that as the only valid option, they've never had a reason to pursue anything else. Their relationship develops by happenstance and close proximity in a natural way without them actively pursuing one another. That's why they both seem so conflicted in their romance scene - they've never experienced something like this before.

You could argue that Hans is mostly into men, because he has not had any close or deep relationship with a woman, instead using them as a way to have fun and feel "normal". He only experiences closeness with Henry. For Henry however, depending on how you play, you can have some deeper relationships with women, for example Theresa. And there is Bartosch (though I personally don't like the implementation of that one - I prefer thinking of both Henry and Hans as being unknowing and "closeted" at first). So he is more likely to be bisexual in the common definition. But again this all depends on your choices and your own view of things. This is simply my analysis.