Mainstream Leftist ideology is all about equalizing outcomes. Anything that is good must be brought down, anything that is sub par must be raised up. Traditional marriage where both husband and wife live happily ever after with their 2.2 children and white picket fence is considered objectively good, so it must be destroyed. Any activity or designation that paints the individuals as deranged/weak/lesser inherently makes the individuals the underdog, the victim, therefore they must be celebrated and raised up.