Some random thoughts on this topic

1) It will be a very painful experience if you have a chronic 精神出轨 without a 肉体出轨 (to the same person);
2) It does not worth doing a 肉体出轨 if you don't have a real 精神出轨.
3) I don't support the idea of 出轨 just for the sake of 出轨. And If you have a "pure" 肉体出轨 without 精神出轨, it still suggests a lack of love to your own spouse, although not necessarily a disappearance of love.
4) Some peolpe may enjoy an open relationship, some may not. It has to take both parties to agree on and accept--without hard feelings---this structure, otherwise it will be unfair to the party in the marriage contract and will soon or later destroy the relationship.
5) It is hard to define 精神出轨 but it is pretty easy to define 肉体出轨. Only if a 精神出轨 becomes relatively persistent (sticky), the impact could become serious. Unlike 精神出轨, any sort of 肉体出轨 is a 肉体出轨 and you have no way to cover it by saying I was just joking/kidding.
6)Whether it is severe or not, depends on who you are talking about. Severe to yourself or to your spouse?
7)肉体出轨 or not is controllable. It is basically out of a person's control if you fall into the fate of 精神出轨.
8)Still I guess it is better following your heart, 出轨 or not 出轨, and in the same time, be prepared to take the consequence that might (or might not) come along. If truth is too hard to swallow, hiding truth is a better option between two bad ones.
9)Well, strive to keep the train run on the right track if you can. 出轨 is dangerious.
10)Cheating others is morally wrong, so is cheating yourself.
11)Marrage is about making a compromise when needed.
12)Love is a trade for happiness.

That said, I believe everyone has his/her inner struggle, which bears the most signifance to one's life. Don't think too much, just live.