The first thing that should be pointed out is that there are no 100% sure ways to avoid pregnancy or disease when you have sex with someone. Birth control is not 100% effective and the only way to prevent disease currently is with a condom and we all know that those break.
I think the case for monogamy, though, is largely one of establishing a type of social bonding that gives order to our natural desire to reproduce. Has anyone ever watched a documentary on lions, for instance? It's been observed that a male lion will kill a lioness' cubs from another male to cause the lioness to go into heat so the new male can spread his genes instead. This is not to say that a widespread breakdown in monogamy would result in infanticide on a mass scale, but it would put every male in competition with every other male where the propegation of the species is concerned. You can further argue that it would cause males to view each other primarily as a threat, even more so than already happens, and it would cause them to view the children of other men in a similar way. This would bring down society as a whole.
Now if anyone has an alternate system that avoids the problems of putting men in competition with each other, I'm all ears. Polygamy would be one such system, but then there aren't enough women around to make that work fairly, so on a widespread basis, you're alienating large groups of men. You can strongly argue that such a system is not fair to women, either.
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