Why is it illegal to have sex in public when it should be just as natural as kissing someone?

Indecent exposure. It's illegal. Also, children are everywhere and minors do not need to see people with a penis and vagina, or same sex individuals engaged in sexual arousal activities. And, quite frankly, I am not interested in seeing people having sex either. Sexual activity is a personal and private matter and should not be on public display. That is what pornography is for. So, if you want to watch people having sex, then buy yourself some pornographic magazines and pornographic videos. There are actors and actresses who play out pornographic sexual acts to be viewed by interested parties. Besides which, a result of sexual activity includes body fluids. Do you really want to sit on a park bench where people left their body fluids? It is a public health risk to have people exposed to fluids produced during sex in public places. Think about that.

It's 'natural' to be naked as well, but there are trade offs that we make. We give up some things that are considered 'natural' as a compromise to being civilized.

cause noone wants to see it

Called inciting public indecency or behaviour likely to cause a breach of the peace

Since when is sex a clean activity? Public indecency aside, why does it make any sense to permit people having sex, sweating, and having body fluids end up in public surfaces that others use?

because you don't KISS with your D.I.C.K. and P.U.S.S.Y.

You're naked... dry humping is allowed though I believe

I know and i want to take dump right when i feel it in the exact spot I'm at.

It may be natural but we're not wild animals who should just be au naturale all the time. We also wear clothing and wipe our butts and shave off our body hair, do other stuff that's not "natural" but it's appropriate in society.