I don't know any good word for this in U.S. English, even though it's common. We would just say "someone who always is talking about sex." As others have said, there are words and phrases for people who are devoted to _having_ sex. Some are "Sex addict," "sex fiend," "nymphomaniac," (female), and (old-fashioned and outdated but you will see them in books) "Romeo," "Don Juan," "Casanova" (male) or "adventuress" (female). "Pervert" might be defined as "someone who is interested in a form of sex that I consider immoral or disgusting." There is a phrase "pimple-faced adolescent." This often means someone who talks about sex, because the age when people are likely to get acne is about the same age when they begin to learn about and talk a lot about sex. It's mildly derogatory and emphasizes the typical annoying characteristics of that awkward age.

Usually someone like that is called a "pervert." You might also hear "sex fiend," but that's not quite as common.

When someone keeps forcing the topic on you: creep, horny, fuccboy.

yes i just read about it on google. thanks bro for sharing knowledge

I didn't know that there was a male term for 'nymphomaniac', but apparently it is 'satyromaniac'! Both are from Greek Mythology. Satyrs are hedonistic goat/human hybrid creatures and nymphs are beautiful deities found in nature :)