I'm a Pro-Marriage Counselor. If a spouce experiences a "crush" on a third person outside of the marriage it's either a strong or a weak crush.
If it's a weak crush, you can just decide not to have it anymore, for the sake of your marriage and family. It can be ended quickly by thinking about how a crush leading to an affair would devistate your family.
If it's a strong crush then it's harmful to your marriage because:
1) Core emotional needs are not being met inside the marriage; and/or,
2) Manditory proctective boundaries have not been set up inside the marriage.
If you're lucky enough that the the third person does not reciprocate (8 out of 10 spouces have sexual affairs, that start with socalled "inocent flirting"), then you can use the "crush" as a learning oppertunity to strengthen the marriage, through learning/implementing basic relationship maintenance skills.
3 people can't but their spouce first!