Because we don't have a strong hope of a good future so instead of feeling like you're building a good life for yourself and a family, it feels like you're doing the same amount of work for a few years of sex and someone who is always going to be a potential threat (divorce, cheating, etc) not because they're a woman but because that's what the overall cultural pressure pushes them towards and the social contract that would protect you from that has been inverted to harm you instead. It's an effect of demoralization. That's why it's a chore for me anyway.