Dirk Greineder took the stand in his murder trial Monday, testifying through tears about how a sexless marriage drove him to a secret obsession with pornography. But the respected Boston doctor also said that this hidden life meant nothing compared with his family.

"What the family meant to me, and means to me, is so much more," sobbed the 60-year-old doctor, explaining why, despite a happy life at home, he maintained a secret life of phone sex and Internet porn.

During his baleful 45 minutes of testimony, Greineder seemed frail, nervous and, more importantly for his defense, incapable of committing the brutal murder of his wife of 32 years on Oct. 31, 1999, in a wooded park near their Massachusetts home.

But his testimony might not be enough to overcome the mountain of physical evidence the prosecution built up against Greineder during its three-week case.

Greineder, who is charged with first-degree murder for the killing, says he briefly separated from his wife, Mabel Greineder, during a walk on the morning she was bludgeoned and stabbed to death. His lawyers say the police failed to consider the possibility of a serial killer, noting that two other people had been killed in nearby Massachusetts parks around the time Greineder's wife was slain.

The prosecution, however, claims Greineder needed his wife out of the way to continue his secret life of porn and prostitutes.

Greineder told the jury that his sexual lifestyle began in 1990 when his wife, Mabel, stopped having sex with him.

"Around that time she seemed to progressively lose interest in sex, and apparently developed some discomfort and pain, which led to our stopping having sexual relations," he said. "I was disappointed."

Greineder turned to printed pornography at first but then found phone sex numbers in the back of Penthouse booklets. "What you do basically is call those numbers ... and they seem to connect you with someone that is supposed to meet that request," Greineder explained, adding that he would call the lines several times in a two-week period then not at all for several months.

The advent of the Internet, testified Greineder, changed his habits. "I became aware that there was a fair amount of explicit sexual material available, and I started looking at some of those sites and logging on to some of them," he noted.

During this testimony, Greineder looked downward and sometimes glanced gingerly at the jury to catch their reactions. The soft-spoken doctor explained the phone sex and Internet porn businesses — switchboards, credit card billing, screen names, and chat rooms — in a meticulous, almost technical, fashion.

But when defense attorney Martin Murphy asked him what he felt about pursuing these activities, he admitted, "It's embarrassing," and then echoed, in a raised voice, "It's embarrassing."

When Murphy began asking Greineder about his family, the doctor spoke more openly. "There was so much mutual warmth and growth in that family that ... I just felt really blessed to have what I had," he testified.

Asked to explain why he felt happy when he was visiting explicit phone sites, he said, "I guess I was gratifying a secondary need. Obviously I did it and I'm not proud of it. But if I'd had to choose [between that and my family], that would have been easy ... In hindsight, it seems so silly, so immature."

The gaunt doctor, who lost 30 pounds in prison and barely filled out his navy blue suit, broke into tears. "My wife was the most wonderful person I ever met. She made me a better person. She helped my family become what we became. She helped me become a better father."

At this point the three Greineder children, who have sat behind their father throughout the trial, began to sob, holding each other for comfort. The jury, who seemed generally nonplussed during the beginning of Greineder's testimony, were much more attentive as his distress became palpable.

As he came off the stand, Greineder's lawyer, Murphy, touched him gently on the shoulder in reassurance.

Earlier in the day, Kirsten Greineder, 30, the oldest of Greineder's three children and the defense's first witness, testified that her family was anything but dysfunctional.

"I truly had wonderful parents, and continue to have wonderful parents," said the medical resident, who attended Harvard Medical School and was a Fulbright Scholar. "I had a great, fun childhood ... a truly rich childhood."

Dressed neatly in a gray blazer, a patterned blouse and a braided gold necklace, she spoke with confidence and conveyed an educated, practical presence. As she talked about her childhood — competing with her siblings on a swim team, both parents in the stands watching — she smiled proudly and appeared well aware of her importance to her father's case.

Kirsten Greineder also told the jury that, after she left to pursue a career in Michigan, her parents maintained a good relationship. They went to the theater, symphony and movies. "I was actually very reassured by the fact that they ... were enjoying activities together," she testified.

When the questions turned to the day she found out about her mother's death, however, Greineder became sullen.

"The first thing that flashed into my mind is that they got into a car accident," she told the jury. "I told [my fiancée] 'I can't go to Boston and spend two hours in a plane without knowing.'"

She then described arriving at her parents' house. "My dad looked distraught. He looked exhausted. He looked like I've never ever seen him before in my life. He actually looked so horrible to me, physically, that I actually felt worried for him. The first question I asked him was 'Are you alright?'"

Greineder also helped to frame the defense's explanation for what happened the morning her mother was murdered. Her father claims that he and his wife separated during their walk that morning because she was experiencing back pain. Kirsten Greineder confirmed that her mother had back troubles.

She also tried to account for her father's preoccupation with getting home to take care of the family dog right after his wife had been murdered. "Our dogs were an integral part of our family life, as much as animals could be," she told the jury.

And perhaps most importantly, she described her father's demeanor when she first saw him after the murder. "It was very much a blur," she said. "He said to me 'Kirsten, I was shaking so much, I couldn't even tell if she had a pulse. I tried to pick her up but I couldn't. I just couldn't do it.'"

During his cross-examination, prosecutor Richard Grundy questioned Kirsten Greineder about the "presence of German literature" in their home and eventually asked about whether Adolf Hitler's manifesto, Mein Kampf was on the family shelves, perhaps trying to establish a darker side of Dirk Greineder.

Earlier in the day, the prosecution called its last three witnesses.

One testified about performing an autopsy on Mabel Greineder. Another told the jury how he used a sophisticated laser measuring system normally used to make highways to create a computer model of the crime scene. The last, a document examiner with the FBI, testified that she believed two Ziplock baggies found near the crime scene were from a box found at Greineder's home.

During the explicit autopsy testimony, Greineder turned toward his children, mouthed "I love you," and shook his head slowly. The same testimony spurred Mabel Greineder's sister to leave the courtroom.

Greineder was scheduled to continue his testimony Tuesday at 8:30 a.m. ET.

The trial is being broadcast live on Court TV.