What is usually meant by 'safe sex' is sexual activity where precautions, e.g. the use of condoms, are taken against the transmission of diseases, notably HIV/AIDS. Earlier, in the 20th century, there have been other interpretations of what was meant by the term; for example, 'the avoidance of sex, notably for young or unmarried people' and 'birth control methods'.
The term as we use it now, i.e. the use of condoms in order to protect against sexually transmitted diseases, originated in the US in the mid 1980s following the rise in the number of cases of HIV and AIDS. The term was widely used in the press from 1984 onward; for example, this extract from an AIDS-awareness campaign, reported in The Daily Intelligencer, January 1984 :
"The goal is to reach about 50 million people with messages about safe sex and AIDS education."
References to the earlier, 'avoidance of sex' meaning date back to at least the 1930s. here's an example from The Stevens Point Daily Journal, March 1939:
"For general health and safe sex habits, children of the same sex or different sex should have separate beds."
[Children who weren't of the same sex or different sex could presumably make their own arrangements.]
This point of view persisted through the 20th century and continues to do so, although the inclusion of the quotation marks in this piece from Hans Sebald's Adolescence: A Social Psychological Analysis, 1968, suggests that the term had already another common meaning by the 1960s:
Analytical observers concluded that this type of dancing was "safe sex" for the teen-agers.
It may be that Sebald was drawing a distinction between his abstinence meaning and the birth control meaning. That was in vogue by the 1960s; for example, this piece from the Vermont paper The Bennington Banner, March 1966:
"Sex outside the marriage bond is condemned, not for the naturalistic reasons put forth by some liberal Protestants, but because it violates the high, Christ - and - church analogy. Thus, the use of birth control devices to permit 'safe' sex before or outside of marriage is similarly unchristian."