Monogamy is not normal sexuality. Waiting til marriage for sex is not normal sexuality. This is a two way street. Whatever "normal sexuality" is or was doesn't matter anymor. Open your eyes and get over it. Homosexuality is normal sexuality now.
You may be making a "scientific" argument but you're basically just repeating the typical homophobic "slippery slope" argument more eloquently.
Maybe something like transsexualism is technically a "disorder" but who gives a fuck. If a guy feels like he should be a girl, then why not let him be a girl? It's a victimless act.
Having sex with children and animals is not victimles, however. That's the difference.