Is there any sex act in which I'm more likely to get an std?

Yes. The answer is, any sex act that does not include a condom! protect yourself, every time!
Yes. Yes, self masturbation. Thats pretty much it. If you have sexual contact with someone else, then you are at risk. Yes oral sex may be less risky for some things, like hiv, but for others like gonorrhea or hpv its still risky. Practice safe sex (condoms, etc..) always and keep safe and get tested frequently!