Don't believe everything you hear.

many people say that they would never engage in sexual activity with a LG, and that doing so would be wrong. my question is, is there any exception to that?

Every reasonably-selected example would be an exception, thanks to the qualifier.

if i am 25 and i get to know a girl who is 13, befriend her, hangout and then eventually get to a point where we are making out and cuddling but no sex, in your opinion is that harmful to the girl? now like i said i'm not asking if its illegal or if its morally wrong but do you think that it would be harmful to the girl?

Fuckin' hell, nigga. Illegal in a few backwaters, but morally right and completely a harmless friendly gesture, as long as you honor your partner.

like i said i am very interested to see peoples response...pro or con, if you think i'm wrong please tell me, i'm always trying to sharpen my opinion!

Sharpen your abilities to engage in debate against the conventional fallacy, and you will sharpen your philosophy admirably.

As far as sharpening things goes, your main flaw was that you held back a bit too much. You could've qualified your counterexample much less and still had a solid position in damn near every framework of ethical philosophy known.