It has absolutely nothing to do with having any desire to fuck one's own daughter. The point is that end result of all your effort and investment is simply that some man enjoys having sex with her more, which means you're basically spending a huge amount of time, energy and money in the service of men's sexual enjoyment rather than anything that makes your own life better. This is an extreme form of cuckoldry, where you're putting decades of effort in to increase the likelihood of something bad (men fucking your daughter) happening. It's like deliberately bringing more lunch money to school so when a bully steals it they get more money.
Investment in a son isn't worthwhile because you have sex with him or something - it's worthwhile because making your son better means he will be more successful, he will have a better choice of women to become his wife and provide higher quality grandchildren and so on.