Is a sexual betrayal of trust more serious than another betrayal of trust? Isn't it the loss of trust, by whatever means the problem?
It's a character flaw. If you can't be honest enough to stay faithful in a serious relationship, not including marriage, you can't be trusted period. I know what I'm talking about here.
Someone put it very well in another thread: Make sure the person you decide to marry won't bail out too easily. Relationships are a lot of work, and there are inevitable and serious problems. The people who are stellar successes are those who work at making it work well.