Short Answer
Islam greatly insists on the privacy of intimacy between any married couples. All acts of intimacy, or of sexual nature, or that lead to arousal, are haram (unless it's with your spouse)
Long Answer
Let me tackle this with subpoints:
When 2 people are kissing on the screen, it's almost always a haram relationship (bf/gf). Watching such relationships over and over again will contribute to culturally re-programming the viewer, and gradually make the viewer adapt to such a lifestyle. That's why nowadays we have bf/gf relationships all over the world, including China, and even Saudi Arabia, when 100 years ago such practices were strange and not practiced neithere in the Chinese or Arabian culture (also Haram).
When your parents don't watch such scenes, I'm pretty sure they are "grown up". The reason they don't watch such things is that because they realize what's acceptable and what's haram. They have been raised this way, and our grandparents, usually, are far more commited to religion than us; they did not have internet, they did not have TV, they did not have random people talking about religion whose credibility are doubtful. I invite you to trust the values and moralities that Muslims have transferred every generation to its successor, and always doubt the new "trends" that appeared the last 50-100 years.
Did you ever see or hear about pious Muslims kissing in public? Forget about Muslims, did you ever see or hear about pious religious Christians or Jews kissing in public? Why won't they do it? Did Muslims do it during the last 1400 years? Then why? If it's haram to do something in public, that means it's also haram for other to see you doing it; which is why you are only allowed to do it in private but not in public. Islam greatly insists on the privacy of intimacy between any married couples, and kissing definitely is an intimate act.
Last but not least, Islam invites us to chastity, to virtue. Islam glorifies the relationship between the couple when it's within a lawful marriage. Sexuality is a natural instinct Allah has placed into all human beings. It's like a tiny flame inside the human soul. Watching such scenes of intimacy, is like pouring gasoline on that flame; It burns you inside. When you're married, you are constantly pouring gasoline on that flame, in a halal way, with your spouse; you won't even care about such scenes; but when you're single, you will not find any way to let the flames out of your soul, so you will burn inside; BUT if that fire keeps burning you for long, you will eventually seek to let its flames out..... the HARAM way.
I am not a Sheikh; I followed a logical approach to answer your question, but if you want a more religious answer from the Hadith and the Quran, you need to go to the nearest mosque.
Actually, learning your religion from the internet or stackexchange is very bad. Some people may purposefuly mislead you. BEWARE. Always visit your nearest mosque.
Salam Alyekum
