One of my problems is that it is a complete non-sequiter. Family relationships psychologically prevent this thing, for example if you grow up from a young age with an adopted sibling then even if you know that they are adopted there is still a massive psychological barrier. This imprinting I suppose you could say is possible of friends too. If you are raised from an extremely young age with someone even though they might fit exactly with what you desire as a prospective spouse chances are you are not going to think about the person in that kind of way.Like homosexual marriage for example.