I'm not sure whether being a gay Christian is valid, but here are some reasons I've heard:
-For example, you can call any part in the Bible 'metaphorical' and get away with taking whatever parts you want literally.
-There are also people who say that being gay was only a sin because there was no concept of legitimate gay relationships back then, and 'gay acts' were between people who weren't actually attracted to each other, they just wanted to get off (and lust is a sin), but legit gay relationships are fine.
-Finally, they can say that the Bible was written over a thousand years, and that the authors shoved in their own beliefs rather than giving an accurate account of our past + what God actually believes.
-They haven't read the Bible