There are LOTS of different kinds of sex and different ways of having sex. It can be a pretty broad term, and can mean different things to different people. However, essentially, sex is something you engage in by yourself or with a partner that makes you feel sexually excited. So, because what a person is aroused by varies depending on who they are, and even on their mood, you can see how different people would engage in sexual activity in different ways.
If you haven't already, you may want to check out these links, just to get you started:
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