Did Marriage Talk Break Up Justin Timberlake & Jessica Biel?

By Michelle Tauber

UPDATED 03/16/2011 at 10:55 AM EDT • Originally published 03/16/2011 at 07:55 AM EDT

Friends of Justin Timberlake and Jessica Biel have already shot down rumors that the couple's recent breakup was caused by cheating.

Nor was it caused by a discussion of marriage, friends of the two tell PEOPLE.

"The topic of marriage wasn't the catalyst or final straw that broke up the relationship," says a Timberlake friend, responding to rumors that Biel, 29, pressured Timberlake, 30, to rush down the aisle.

PHOTOS: Justin & Jessica: Their Romance in Photos

Contrary to some reports, "there was no ring shopping," says a source close to Biel. "She wasn't even thinking about marriage. That didn't play a part in their breakup at all."

In the wake of the split, Timberlake "is upset," says his friend. "They always spent vacations together. They always spent down time together. He was invested in her, in her friends and family."

Continues the friend: "He was invested in their relationship on a lot of different levels. After four years it's just not easy to walk away from it."

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For more details on their breakup, pick up the latest issue of PEOPLE, on newsstands Friday