18+ question about relationships.

Is it common for partners to fantasize about sex with other people while in a relationship? Is it okay? Would someone be an asshole or insecure for not wanting their partner to do that and even breaking up if they found out they did?

Example. A woman finds out her husband has threesome fantasies. Most people would say a fantasy isn’t always an actual want/plan to do it and it’s not the same as cheating, so it’s not a problem. If the woman is upset or leaves him she’d be an asshole. Is she?