Here is the CDC's STD trend report from 2000 You'll notice every single major STD was trending down.
It was when the abstinence was pushed instead of safety and when protection and barriers were deemphasized following the 2000 election that rates began to rise again, reversing decades of progress.
http://www.cdc.gov/std/Trends2000/Trends2000.pdf
In contrast, here is a graph from the 2004 report - you can already see the effects. This is pattern repeated throughout the data. I'm sure the abstinence education folks are well meaning but the end result is more sick teens, more unwanted pregnancies and more abortions.
Now there is blame to be shared - the gay community in particular has moved away from emphasizing barriers. Here in Atlanta I have actually seen "bareback parties" advertised. It would not surprise me to find that is also a problem California. Many of these men are also active with girls or women so it effect the larger population.
But in the end - one of the cheapest things we can do to reduce healthcare costs and abortions is to make barriers freely and widely available.
First, your chart shows nothing about causation; even the suggestion of correlation is weak.
Second, the study I referred to, published 2 months ago in the Californian Journal of Health Promotion --- the one reporting 1.1 million new cases of sexually-transmitted infections among young people in California in 2005 --- is certainly underatated. Study author Dr. Petra Jerman told Medical News Today that the statistics revealed an epidemic of which, like an iceberg, only a small part is visible.
The authors acknowledged that their figures are underestimated because of incomplete screening of sexually active young people, and failure to confirm the effectiveness of treatment through follow-up testing.
Moreover, their figures reflected infection rates for chlamydia, gonorrhea, HPV and HIV, while yours, unless I'm mistaken, are only for syphilis, by far the least common of the STI's.
I assume that if the full range of STI's are underreported in California, as Dr. Jerman says, they're even LESS accurately reportd in other states, many of which don't even have uniform county-by-county reporting requirements.
Here's a world map of adult HIV prevalence. I would hesitate to make any generalizations based on the map, but I will note one thing I don't see: any consistent correlation between countries with the lowest rates of HIV infection (green on the map) and a high prevalence of the type of "modern, comprehensive" sex education pical of Europe and the USA.