Under Aristotle an action is immoral if is goes against a final cause or purpose of either the action or any of the beings involved in the action. To make the case that any sex outside of the sex act that does not include any artificial means of preventing conception is immoral, Feser relies on the idea that the final cause of sex is conception. He goes on to state that nature of the sex act being conception, it should not be done outside of marriage, which is the institution whose nature is raising kids. Because humans are driven to copulate often, he concludes that married couples should have lots of kids.
Obviously my summary leads out the details of his arguments for his conclusions, but his arguments all rest on the definition of morality. The problem is that this ignores what the consequences might be to humans from these actions. Because advances in technology have over come certain obstacles humans have faced through out our evolution, certain parts of our nature if left unchecked could lead to suffering for humans.
First, let us deal with the idea that going against the nature of an act is immoral. Previously I discussed walking and how a treadmill frustrates the purpose of walking by not allowing us to go from one place to another, as is the purpose of walking. Consider another example, that of walking by putting one foot in front of the other so as to cause you to trip. Is this immoral? If it was done unintentionally, we would say it was silly or foolish but not immoral. What if it was done intentionally? If I trip over my feet as a means of entertaining as for example a comedian or actor, is this action immoral? Certainly not. In the case where it is meant to entertain we would not even say that it is silly or foolish.
Now let us consider what morality actually means. For an action to be immoral it would have to promote the bad, it would have to have a bad or negative consequence towards humans and that consequence would have to be known and probable. Non-procreative sex does not meet that standard. However, the notion of everyone having lots of kids does. If everyone procreated as much as they are biologically compelled to, we would have a population explosion. We would deplete our resources at an enormous rate and soon would not have enough to feed everyone. While technology might be able to help us to delay the mass starvation, we would inevitably consume all available resources.
There are 2 possible solutions to this problem, both of which go against the nature of humans as determined by Feser. The first option is the road we are currently on, using birth control to reduce the number of conceptions and keep population growth down. The second option is voluntary celibacy. It is unreasonable to expect that the population will be able to suppress their sexual urges enough for the second option to work. When under sexual arousal people tend to make poorer decisions then when not aroused. This conclusion is based on a study by Dan Ariely documented in his book Perfectly Irrational. The only option that does not lead us to increased human suffering is to support birth control.