Why would a guy want to finish inside you?

It's an incredible sensation.

Because it's the natural way of sex. Use a condom.

Cause that's hot af. All organisms are made to reproduce, that's the bottom line. A bull/lion/zebra doesn't go around pondering such things, it eats, sleeps, and breeds, meaning there's a part of every organism's brain that tells it to "finish inside you". To that end, the ultimate goal, for any animal, is to "distribute their genetics successfully", which is backed up by the fact that reproductive organs require positive arousal, so that the organ can complete it's function, so to say. So while you may just be having casual sex, the main purpose of intercourse, as it pertains to biology, will always be for the involved organisms to fully utilize those organs for the purpose of reproduction.

It feels good

Why would he not? We're genetically wired to have sex and reproduce, and that's part of it. Use condoms- or get on birth control.

To knock you up. That finish is the way we do that you know.

He a nub