Unless she happens to be rather unconventional, it's a safe bet that the bride-to-be expects her new husband to be faithful to her for the duration of their marriage. Some men simply can't handle the concept of having only one sexual partner for the rest of their lives. They imagine that they'll get bored eventually and sex will become monotonous. Or worse yet, they'll have no sex at all. Sometimes life simply gets in the way, especially once you have kids.
Many biologists claim that monogamy as a general rule isn't "natural." There aren't many other species that practice it, and there's a good biological reason for this: Men don't have to carry and take care of babies. And while women are born with a finite number of eggs and release just one per month, men constantly manufacture sperm and release millions of them with each ejaculation. It's in their best interest, biologically speaking, to spread it around for the best chance of furthering their line. Of course this is no excuse for infidelity. If a man leaves his fiancee at the altar because he's worried about losing his sexual freedom, there's a possibility that he would've cheated eventually had he gone through with it.