Keep in mind that the original movie was rated PG. But, this was before the MPAA even came up with PG-13. I think that if PG-13 was around when the movie came out then that was the rating it would have gotten.
The scene with Josh and Susan kissing was much more graphic (not the best choice of words here. But, whatever) in the movie then in the show. I feel that in the show the only reason why they were kissing and hugging as long as they were was for the sake of the length of the song. Had the song been shorter then it wouldn't have gone on as long.
But, homeimp, let me ask you a question. We both know that the whole point of Big, both the movie and the musical is that it's this teenager who is both feeling emotions and going through things that are totally above him. Now, I would add him having sex with Susan (once again, I feel that in both the movie and the show it is implied) is one of those things that he did not really being ready for it. Would it not have bothered you if it wasn't a 13 year old trapped in the body of a 30 year old?
But, as I said, in the movie, Susan unbuttons her shirt. Yes, she is wearing a bra but still Josh touches her breasts. I am VERY laid back when it comes to things like censorship etc, so I don't know. But, I could see why some parents might find something like that uncomfortable for their kids to watch.
I remember reading Big: The Diary Of A Broadway Musical, and there was a quote from someone in either the production or creative team. The quote was talking about how parents were taking their kids to see Beauty and The Beast as their first Broadway show, and they felt that Big would open up another option for first Broadway shows. Keep in mind that when Big was playing there was only one Disney show and parents only had one option. Clearly what they wanted worked, at least for me. Considering that at 6 my first Broadway show was Big.
Also homeimp, I can tell you after having recently gone to the archives to watch the Broadway production. The scene with Josh and Susan comes off MUCH more tasteful and family friendly onstage then it sounds on paper.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll