Neither "declining population" nor "lack of young people in relationships" indicates people not having sex. It only indicates people not wanting families or kids.
Right, but those aren't the only two signs we're talking about. If people are having lots of sex but don't want kids, we might see that in booming contraceptive sales. If people are having lots of sex but not using contraception and not having kids, well that would show up in wider culture too (e.g. gay dating sites would proliferate). Sex on a personal level is a very private thing for many people in many cultures, but trends can always been seen because there is always a commercial side.
