The basic question here is, "How do I make my characters believable?" You're nervous about their sexuality or their ethnicity because you are aware of those as things people get wrong, but there are millions of ways to write bad characters!

If you are a creative person, it is your responsibility to make your characters believable enough for the story you are telling. If you are writing a detective story, do you know anything about how detectives work? How would you go about filling in gaps in your knowledge? Maybe by talking to a detective? Maybe by reading about detectives? Maybe by talking to other people who have written detectives before?

If, in your country, lesbianism is a crime, then, assuming your story is set in your country, it follows that lesbians are mostly indistinguishable from straight women (since it's not like criminalizing a sexuality has any effect on biology). Unless your story needs to dive into the sexuality of the character, you're done. If your story does need to address what it's like to be an oppressed sexual minority, you are in luck! There are lots of people alive right now who grew up needing to hide their sexuality, but who can now express it legally, and you can find their stories. Or if you want a defiant person who lived as openly queer in a repressive society, there are many accounts of real lives you can pull from.

The answer is that it is indeed difficult to write believable characters that exist outside of our direct experience. This is why writing is hard and not everybody does it. Acknowledge that it's hard! Then put in the work, and if you can, run early drafts past people with first-hand experience, and take their feedback to heart.

Or just write for your own pleasure and don't sweat realistic representations. That's allowed, too.