This is why some men struggle with erections during sex

Anything that interferes with sexual arousal and/or the physiological process that causes erections can be a boner-killer.

Many men, especially older men with cardiovascular problems, have difficulties with erections, or erectile dysfunction. This is largely a plumbing issue. The physiological process required for erections stops working properly. In other words, not enough blood gets into and stays in the penis. For these men, erectile dysfunction drugs such as Viagra and Cialis can be helpful, as they amplify the erection process. But sexual arousal is still required. That’s why a guy can take a pill but not get an erection until he and his partner hit the sheets. Sexual arousal is needed to kick off the physiological erection process.

Large quantities of alcohol, some recreational drugs, and many medications can also negatively impact the physiological process necessary for erections. However, when you hear of young guys struggling with erection difficulties, the culprit is almost always anxiety.
Anxiety is an emotional and physical response to real or perceived threat. The threat can be physical of psychological. In the context of sex and sexual performance, it’s psychological.