Of all the ideas I grapple with, I think that morality and sexuality challenge me the most.
The latter is both the easiest and the most difficult to explore. I understand the sex drive on a logical, biological level, as an imperative to perpetuate the species. At the same time, it challenges me on a personal level.
My sex drive still exists, but I do not have the familiar anchors others have: no emotion appends to it. I do not have any sort of preference to express as others do, not based on sex, ethnicity, body type, or other superficial features such as hair color or eye color (note the Western cultural obsession with "blonde over blue," for example). Despite my rational, fact-based understanding of it, sexuality does not bend to rationality. And so the drive exists, though nothing may be done about it aside from the purely mechanical.
Morality, however, is another creature entirely.
So far, every bit of philosophy I have read, every attempt to explain the need for a moral code, ultimately appeals to emotion. Such appeals suffer from a certain lack, to me. It requires, at base, one recognizes a certain "good" or valued state of being that cannot be explained entirely rationally. Various justifications are essayed to define such a "good," but they always relate ultimately back to some kind of emotional appeal.
Even the most rational-seeming system I have studied, Kant's Categorical Imperative, appeals to certain things which cannot be defined without some emotional basis. The first and most well-known form of the Categorical Imperative goes: "Act only according to that maxim whereby you can at the same time will that it should become a universal law." On first glance, this seems logical. But only if you accept a few existing presuppositions.
The first is that one must already value the integrity of society. Kantianism requires society and social cohesion to be valued for this to work, else a person can use the Categorical Imperative to justify any act. The traditional example to demonstrate the Imperative goes thus: if you cannot will that everyone lies when it is convenient, then you should not lie when it is convenient. But that supposes a moral value on the state of affairs where people do not constantly lie. Where does such a moral value come from? Kant suggests they come from various duties that we have as humans. Whence do such duties come, then? How do you define a necessary duty? You cannot. They are tautological, where they are necessary because they are good, and good because they are necessary. When one questions the basic definition of "good," then it breaks down.