sex act vs sexual practice

sex act and sexual practice both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex act	Yes	No	No	No
sexual practice	Yes	No	No	No

As nouns, sexual practice is a hypernym of sex act; that is, sexual practice is a word with a broader meaning than sex act:

sex act: sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur
sexual practice: activities associated with sexual intercourse

Other hypernyms of sex act include sex, sex activity, sexual activity.

sex act (noun)	sexual practice (noun)
sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur	activities associated with sexual intercourse