safe sex vs sex activity

safe sex and sex activity both are nouns.

Word	Noun	Adjective	Verb	Adverb
safe sex	Yes	No	No	No
sex activity	Yes	No	No	No

As nouns, sex activity is a hypernym of safe sex; that is, sex activity is a word with a broader meaning than safe sex:

safe sex: sexual activity (especially sexual intercourse) with the use of measures (such as latex condoms) to avoid the transmission of disease (especially AIDS)
sex activity: activities associated with sexual intercourse

Other hypernyms of safe sex include sex, sexual activity, sexual practice.

safe sex (noun)	sex activity (noun)
sexual activity (especially sexual intercourse) with the use of measures (such as latex condoms) to avoid the transmission of disease (especially AIDS)	activities associated with sexual intercourse