But yet, that is precisely what your argument does--it argues to the Bible--so they won't listen, according to you. Any who reject what certain authors in the Bible say about homosexuality certainly aren't going to listen only "if God or Jesus, themselves, tell you homosexuality is wrong." They would most likely reject all of Scripture or at least all of what Scripture says regarding homosexuality. And of course you seem to be implying that certain persons' views in the Bible are not of God, which would be wrong.
This is just one problem with your argument. Homosexual acts are wrong whether before marriage or after, so arguing to only sex before marriage confuses the issue and leads you to your fallacious conclusion.
And this is your fallacious conclusion. You do not know this to be a fact and you have no way of proving your assertion. If heterosexual couples will get married without first having sex, then it is entirely plausible that a gay couple will get married without first having sex. The issue is that it is more likely to be religious couples who will abstain from sex before marriage then non-religious couples.
Non-religious homosexuals are just as likely to reject what the Bible says about sex outside of marriage being wrong as they are to reject what the Bible says about homosexuality, so I fail to see the point of your argument.