Right.
That is helpful thank you.
There is some small risk of skin-to-skin transmission of some STDs through frottage, primarily herpes and HPV; but even for those, the risk is lower than with anal or oral sex. Mutual masturbation is essentially zero risk for anything. There is no realistic risk of transmitting HIV, HBV, chlamydia, etc. Syphilis is possible but unlikely. Oral sex is also pretty safe, by the way--but still riskier than frottage and mutual masturbation.
My guess is your friends/partners are telling the truth about their STD status; most people do. But even when people are truthful, of course the accuracy of that information depends on what they were tested for and what exposures they had since the tests were done. Most gay men are not tested routinely for herpes or HPV and many infections are asymptomatic, so history that someone doesn't have those infections doesn't mean much.
Good luck-- HHH, MD