In short Empathy we can mentally insert our selfs in to the act then we would like to feel that way

Sex is arousing in both the general and specific sense of the term. It's exciting. It's a big, important thing for us humans, and it's something we rarely see because its culturally taboo.

From an evolutionary psychology perspective (and speaking very generally), men get physically aroused when they are in a position to compete for sex, and women get physically aroused when it appears likely they will have sex. Seeing other people having sex triggers those responses.

This is also common across other mammals, so it's likely to go far back from an evolutionary perspective and not something that we can rationalize in terms of human behavior.

I suppose it's similar to yawning. If someone yawns and you're a bit tired, mirror neurons in our brain make us experience as if it is ourself

Evolutionarily speaking, video porn like we have now is brand new. In the past, seeing a naked woman meant sex was a legitimate option and men would be aroused by that. Our brains haven’t caught up to the reality that the porn star we see on the screen isn’t actually in front of us ready to get it on.

I get hungry if there is a long drawn out scene of a person eating a delicious looking meal and them enjoying it immensely.

Same idea.

If you saw somebody doing something you thought to be enjoyable, you wouldn’t want to try it out?