"Ummm lets see what would happen if two 16 year olds - who are in a "long term relationship" spent the night. You're parents have every right to lay down rules. It's not sexism. You just think it's unfair. You're 16, and you've admitted to having engaged in risky behavior in the past. They obviously don't want any grandchildren - yet. You clearly haven't proven proper judgment or responsibility in the past. You need to grow up. When you're 18 and not living under their roof, you may invite anyone you wish over.
But neither you nor your brother should be allowed to have the opposite s** spend the night. I'm surprised that the parents of the your brother's female friends even allow it. "