Well, time for me to post my opinion. Sorry for my poor English, by the way XD
Is reading Yaoi a sin? It depends on what "sin": A blasphemy, or the sin in the view of society?
I've talked a lot about this topic with my mother (who's a very devout Christian), and she said that most people mistake it as a form of sin.
First, it is true that in the Holy Bible, it was okay to keep a slave, and women were lower in status while men were higher, but, see the past tenses? That's because those rules apply in the Past Promise, where every evil person was killed and sent to Heaven. Now, we live in the New Promise; where Jesus has suffered what humans suffered and has been cruxified.
I myself haven't read the Bible thoroughly. But if homosexuality is viewed as a sin in God's eyes, then I'd rather be an atheist instead.
Humans sinned so easily; for example, eloping or per-martial sex is a sin. But, actually, just thinking about sex (or knowing how it works) is already a sin.
As for Yaoi or any other homosexuality is viewed by humans, it depends on how they are raised and educated. During the 18th-19th century, the hunt for homosexual men is on its high, the church wanted to eliminate every one of them because homosexual is a blasphemy - while it's clear in the Bible that Jesus said that anyone who hurt someone else in God's name has made a grave sin, because of God, those people had hurt and killed many. Honestly, I almost leaped out of my seat and choked a sex specialist during SE in my class because he said something vulgar about homosexual, but I didn't because he added, "Don't tease or hurt them; they're still human and hurting them is the same as hurting any other human." (Yeah. Be glad you said that. If you didn't, you would have been in a hospital by now for a broken neck, old man.) But, alas, we can't force some people to change their view. (That's why I write a fan-fiction dedicated to stop abuses of homosexual couples.)
Homophobes, in my opinion, are the same as racists; they won't accept some people because of their nature. Homophobes won't accept homosexual (homosexuality is mostly from their genes) people, and racist won't accept some races because they're black or so (skin color is destined by the genes, for black people, at least). And they're the same as sexists, just a tiny bit different.
Sorry for any misinformation, I can't argue since I haven't read the Bible yet. I was just so mad about a certain homophobic person and I need to let it out.