Why is he being left alone with other kids? He needs to be closely supervised.
If he's lacking self-control or doesn't seem to understand the serioiusness of it, not allowing the opportunity for him to do it again would likely help to stop it from happening again.
Are you certain that he understands why it is wrong? Does he really understand what "private parts" are and why he should not show his to others or ask others to show theirs to him? Or could he possibly just be viewing it as a rule he shouldn't break, like no hitting or throwing toys? I'd talk to him again--calmly, but firmly--and explain the seriousness of it. And don't let him out of your sight while with other kids for a while.
Just so long as you know that his behaviour is absolutely normal for a 5 year old, then you can deal with this in a grown up way.
I agree that it is undesirable for young children to be fixated about 'sex', but at this age looking at the bodies of other children and playing kissing games happens frequently and isn't a sign of future perversion. It is natural to show interest in other childrens bodies and if you come from a background where naked bodies are seen as taboo, darn right you are going to sneak a peek when you can! And if you know that it sends your parents ballistic when you do it, it has to be worth another look!
I think the best you can do is supervise your children when they are with others and try to encourage games to be played in a communal area where there is no opportunity to sneak off and play 'doctors and nurses', 'mummies and daddies' and the myriad of other games children play where they look at each others 'privates'. Your son will grow out of it soon enough......until he reaches his teenage years, and then you will have to accept his sexuality, hard though it is for us parents to do. By that stage he will be just as disgusted by you and your husbands sexuality as you are by his, and things will be more balanced! Good luck.
little kids kiss all the time.....they see parents do it, therefore in their little minds, they "love" each other and kiss each other on the cheek....lordy, i wish my mammaw could recall how many "weddings" she attended, with curtains for veils, and sunday dresses for gowns.....and we were all under 7....children aren't really thinking 'sex', they're thinking...ok where's MINE and why is YOURS different? therefore, the other kid asks the same question...show and tell time.....it does need to be addressed in a way that shows it should NEVER happend....however, parents getting together and getting upset about their kindergardeners kissing, is too much for me....one day last year, my stepdaughter told me her "boyfriend" kissed her....(in her town, busses are based on grade, and pre-k to 1st have one bus)....so i asked, fiegning astonishment, "omg, whose your boyfriend, how old is he, where DOES he live, cos i need to see his mamma!!"....she blushed and smiled and says, he's not my REAL boyfriend...and he's in kindergarten(a year behind her)".....so i said "ok, i'm still calling his mom"(i had/have no idea this kids name, but she was buying it!)....so she says, "Jess, he's not my REAL boyfriend, he just likes me, but i don't like him"....so i told her not to "kiss" boys she didnt like....never heard about it again.....but it kinda backfired on me, as her daddy was roughin with her, and he tried to kiss her, and she said "jessie said not to kiss BOYS i don't like!"....df still haggles me about it.....but when i explained, he got it.....idk, i guess to me, them 'looking' the FIRST time, is an icebreaker....now you get to explain why they dontdo it again.....but with a repeat issue, maybe DAD should address this, and rather than screaming "private parts", have dad explain "respect"....that good boys don't look at girls with no clothes on.....