IIN i want to write a lesbian relationship?

That's the job. There's no writers who have been bitten by a dinosaur, shot into space, experienced the apocalypse or lived on a deep ocean research station. Good books about all that shit though. You write the world you imagine not the world you exist in. If you wrote about only what you knew, your book would be called "Look at Me. I'm Writing" and every sentence would be "I'm in a chair. Typity type. I like letters"

No, you cant write FOR women or homosexuals as if you know them, but you just have to know enough to build an experience. If it's a concern, there are forums where lesbians discuss their experience. Theres other books about it written by lesbians. You cant live their life but you can grasp what it is like. And after you write, should you still have this concern, you can ask lesbian women to read it and see if it speaks to their experience.

But you should know...lesbians are not homogeneous. They are not all the same dude. Your writing might speak to one person and not make sense to another & that's the beauty of creating a character. It's a person with flaws. Some people will get it, some wont, but lesbians are not weird unicorns, they are people with flaws.