Or possibly we understood going in that marriage is about learning and working together. Not to mention that a couples sex life, like most other areas of life, is something that a couple can and should grow together in. Every relationship will have it's issues. For some they will be sexual, for others financial, for others lifestyle issues. If you aren't aware of that going in and willing to work at it your relationship, no doubt about it, will fail.
Sex is a big deal in a relationship, no doubt, but it's not that biggest issue a relationship will face. If you can communicate then you can deal with 99% of the "issues" that you will face, sexual or otherwise. If you can't... see the underlined above.