I trust my husband's word more than our friends'.
I wouldn't even say "I heard you're having another relationship" because put that way, it makes it sound accusatory: "I hear you're lying to me". I would put it in terms of "So-and-so said this, why do you think they would believe that?"
My husband's the type of man who has more female friends than male friends, so all the numbers in his phone are either women or family/work-related. He works out of town more than he's home. It would be relatively easy for him to have an affair. But I never would have married him if I suspected he was capable of that kind of deception.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker