There's not a one size fits all. Anyone is capable of anything at a given time, and that extends to acts of kindness and violence and patience and betrayal, among others.
I understand that this question is male-actor-oriented, but if I found out Grandma was gettin' some on the side, I'd first wonder where the problem is between us, because one thing I've learned from watching people is that it's not necessarily about sex, not necessarily about fidelity. Both men and women. People have holes in their psyches to fill, and they're drawn to fill them in different ways.
I've given up on jealousy and possessiveness. They're bad feelings that don't do me any good, and I don't want to have them anymore. No, I'm not saying that it makes it right to step outside of a relationship. I'm just saying that that boundary is a movable line from one person to another, for reasons that run a psychological spectrum.
I know, I know, sex is a Big Deal. Sometimes I wonder if it should be. We can draw lines - if you loved me, you'd look for a better job, you'd have sex with me more, you'd clean the house, you'd run my errands, you'd let me loose for a weekend, you'd let me see my friends, you'd suppress sexual desire for others. Lots of lines there, but sex is the big, solid one for most.
People are complicated. The relationships they form are equally as complicated, if not more so. Humans are not naturally monogamous, and I say that after some 40-plus years of watching human interactions. If someone wants to protest that, yes, they are, I believe them even while keeping the second sentence of this post in mind. I'm not saying infidelity is right. I'm saying I've resolved to not be surprised anymore.