Questions & Answers about Þau tala um kynlífið sitt.
Because the subject is þau, which is 3rd person plural.
The present tense of tala (to talk / speak) changes by person and number:
- ég tala = I talk
- þú talar = you talk
- hann/hún/það talar = he/she/it talks
- við tölum = we talk
- þið talið = you all talk
- þeir/þær/þau tala = they talk
So Þau tala is the correct form for they talk.