Anyone who says marriage is 50/50 is, well, incorrect. Marriage is not always an even distribution of time, energy, and work. Sometimes, it will be 80/20. Sometimes, it will be 10/90. Sometimes, it feels like both people are just as invested, just as hard-working, as their partners. Even in those circumstances, I really do not ever feel like 100% of the time, both partners are putting the same amount of work into their marriage.
I honestly feel like people who get married with the notion that one day their union will be even and stay that way are setting themselves up for a big disappointment. That's not to say that both people can't work hard and tirelessly for their marriage. I just think saying a marriage is a split partnership, right down the middle, is expecting your partner to do things exactly as you would do them and if they don't, you might start to feel like they aren't working as hard as you.
There are so many areas to which a marriage needs energy. Just a few would be household chores, dealing with the kids, making dinner, working, praying for one another, sleeping in, dating each other, this is a very small list. To say someone is giving exactly the same amount as you means keeping score. "I've done all of the housework this week!" "You've got to sleep in the last 3 Saturdays!" "I always have to make dinner!" I am not saying both people shouldn't be pulling their weight. But there will be times that it's just not like that. If it's a constant and unending problem that your partner never helps, that's one thing. But don't expect a give with every little take. Don't expect the other person to give you back exactly the same as you put in. Give with the idea that's what God wants you to do and you will get back in ways you never even dreamed of.