The Dual Mating Strategy of the Swallow
Two things need to happen for swallows. The female swallow needs to get fertilized by the male. Once the eggs are laid, someone needs to tend to the eggs.
Swallows have two reproductive options, which are similar to the ones humans will have. One strategy is to mate with one swallow and guard that nest vigilantly to invest all their eggs into that basket. The other strategy is to mate with as many females as possible and hope that the nests will be taken care of.
The question is: Which strategy is the best? There is definitely a macho element to the second strategy. For the swallow, the answer depends on what the other swallows are doing. If there are many nests being taken care of, the best bet is to impregnate as many females as possible and hope that the eggs are taken care of. On the other hand, if many swallows are using the second strategy, nests will not be tended and females will start to put more emphasis on tending their nests. This is called an evolutionarily stable strategy, which is that the strategy depends on what the world is doing.
These same strategies apply to humans as well. You can stay monogamous and take care of one family or you can go around and impregnate as many women as possible.
Our strategies are flexible over time and it depends. Modern technology, such as birth control and abortions, has affected these strategies. Resources are more abundant, so the chances of a child left alone surviving are higher. The best thing to do depends on what other people are doing and how capable you are of executing one strategy to another.
The point is that there is no one best strategy. Even a simple creature like a swallow has to stay flexible.