When does it become understandable to cheat?

Cheating is never acceptable or understandable.

If you are in a relationship, and want to be with someone else as a sexual partner, you need to get out of the relationship.

What you should be doing, if one person is not interested in the sexual part of the relationship is getting counseling for that problem and working on the entire relationship to make it better.

The simple answer is never. If you are compatible in all other aspects then getting the one topic that doesn't seem to fit on the table and talk honestly about it. Most relationships have different desire rates anyway, and usually one or the other is dominant with initiating sex which is also normal. Having a slower libido doesn't mean you go outside the relationship to fulfill your desires, that is called cheating or infidelity. If the person that has a lower sex drive tells the other to "go have fun" with another person they are only putting a band aid on open heart surgery because your relationship is dead if the person with the higher sex drive does it. No one who claims to love the other would consider having an intimate relationship with a third party. If talking it through, then maybe couples counseling after a thorough medical check up to be sure something medically is amiss and easily remedied with medication. It could be hormonal, or psychological in nature.

As much as your question makes sense and I truly understand the logic of it, the answer is never. What you will be doing is trading in the 90% of good relationship for the 10% of physical. Your betrayal will cost you everything. She will find out and your logic won't change the amount of pain she will feel about your sneaking around and lying and your relationship will be over. What you need to do it tell her up front exactly how you feel and that you need that to be part of your relationship. It's not fair to expect a partner to forsake all others with a promise of love AND physical intimacy and then renege on it. Men need that to keep engaged in a marriage and to feel intimate and women shouldn't get to hold all the cards while men just suffer. It is part of the marriage commitment.

Never. Find out why the other person doesn't want to have sex and fix it. Remember why you fell in love, and work it out. There is medication for these things for both men and women. Romance your partner, put your relationship first, If divorce is not an option, fix the problem before you grow resentful. You are supposed to be a team.

When you are divorced or ended the relationship. In other words not while you are committed to a relationship.

Never. Be adults and resolve the problem together.

Never. Either leave them or honor your vows. Cheating is never justified. You shouldn't have gotten married at all if sex was your biggest priority.

Never! Either you take them as they are - whether they want sex or not, or you don't date them at all. Don't be selfish.

Just grease up, bite your pillow, and let Brucie have his way.

It'll be over before you know it.