If STDs are sexually transmitted how can you get them by touching blood that's out in the open, ?

Termanology. The category of "sexually transmitted disease" is just a group of problems that can pass sexually. The germs involved are not limited to that mode of transmission. Hepatitis B can transmit sexually or through a tainted needle, or contact with blood of an infected person.The same goes for AIDS, syphilis and many other diseases. Some are less likely to pass through non sexual means.

Related Questions

Are STDs transmitted only by sexual contact or by mixing blood? Which STDs are only sexually transmitted?

Theoretically none. But trichomonas would be difficult to get through blood, as would gonorrhea, although the latter is possible on a theoretical basis. It would also be very unusual to get herpes except by sexual activity unless you don't consider oral sex, sex. Read more...
Few. Perhaps trichomonas and gonorrhea, although the latter could conceivably be transmitted through blood under some unusual circumstances. Read more...

Does it happen that people get stis or STDs sexually transmitted dieases from touching stuff?

Unlikely. Sexually transmitted disease, means the disease is transmitted through sex, although, theorically if you touch an active lesion from someone with herpes and then you touch your genital , there is a possibility to acquire the virus this way but i doubt anyone ever been infected this way. Read more...