Holy shit, I'm just going to beat off from now on :fear:
That would be playing it ultra safe, i.e. abstinence.
Alternately, as regards syphilus, you could:
1) avoid skin to skin contact during intimate activities
2) make sure your partner is tested and clean
3) be aware of symptoms so you can ID it early
4) be tested regularly to catch & treat syphilus before it makes you look like something from the tv series "The Walking Dead".
"No home remedies or over-the-counter drugs will cure syphilis, but syphilis is simple to cure with appropriate antibiotics from a physician. Treatment will kill the syphilis bacterium and prevent further damage, but it will not repair damage already done."
"...The surest way to avoid transmission of sexually transmitted diseases, including syphilis, is to abstain from sexual contact or to be in a long-term mutually monogamous relationship with a partner who has been tested and is known to be uninfected."
"...Any unusual discharge, sore, or rash, particularly in the groin area, should be a signal to abstain from having sex and to see a doctor immediately.
http://www.cdc.gov/std/syphilis/STDFact-Syphilis.htm