If limiting contact to kissing and mutual masturbation, is there much risk of STDs? If yes, which ones?
Without any oral-genital contact, the risk of getting an STD is very, very low. You can consider kissing and mutual masturbation to be very safe sex. I think there is one report of trichomoniasis being transmitted from mutual masturbation between women, but I have not heard about that in men. There is a small risk for HPV -- genital wart virus -- from mutual masturbation, too.
To your health,