I think it's just expected. And not without reason. The vast majority of people couple up, whether they do so within some kind of legal or religious framework or just because. We're kind of hard-wired to do it, since the perpetuation of our species depends on it. Animals do it too.
I don't know if Christianity expressly refers to the woman completing the man in that sense, but since in Christian theology, woman was created from a man's body part, man is by definition incomplete and returning her to him (I think it does say they become 'one flesh') is an act of completing him.
It's been a part of the romantic myth for ages too though. "You complete me." This idea that there's someone out there who is a perfect fit for you, like two jigsaw pieces. It's bs, of course. But it's enduring because it gives meaning and purpose to something that is chaotic and random. As fnord points out it's cultural too. In the west in latter years we've had this cult of individuality developing and the idea that another person is necessary to complete you is falling, I believe, out of favour.
But I don't think it's strange for your parents to want to see you in a relationship, as annoying as that probably is. That probably goes quite deep psychologically. Part of parenting, maybe all of parenting, is about giving your child everything they need to be able to live without you at some point. Perhaps your parents see your lack of progress in the relationship department as their own personal failure to prepare you for that.
_________________
It's dark. Is it always this dark?