I've heard that heterosexual porn can lead to homosexual desires. Is this true?
Indirectly. The problem is that sin tends to be progressive. A part of the appeal to sin is the excitement and novelty of it. To illustrate: if you ride a roller coaster, it is thrilling the first time. But each time you ride it, it becomes less so. Eventually it will feel almost boring. This is why people search new and bigger roller coasters. The same thing happens with pornography. The first time it is shocking, but over time it is less, so the person looks for more. Yet, after a while they all begin to be the same, so the person search out more twisted forms, such as homosexual themes, multiple partners, or those involving domination.