Well, I'm an adult, and I understood what the original poster meant. There's is some context required when the word sex is used. When used to describe actions between a biological man and a woman, everyone takes it to mean vaginal sex. Which is kind of a redundant term which no one would actually use in practice.
The preceding conversation would never happen. What sex meant would be understood. Any act other than sex would be specified individually, such as oral sex (like sex, but with the mouth), anal sex, etc. The probably wouldn't even use "oral sex" as a term. They's use something like "blowjob" or "eat your pussy", which further separates it from the idea of sex. Not that they aren't sexual activities. But they aren't sex.
For gay men, the definition of sex differs, but most take it as being about anal penetration. It's a lot less clear when it come to lesbian women, but then again, that's not really relevant when it comes to the current discussion.
And honestly... did you truly not understand yourself what the original poster meant by sex scenes? If you really didn't, you would have talked about all the scenes in there that you saw as sex. Instead, you sought to correct their definition because it was not the one you preferred, even though it's the correct one.
