The sensible response to this question which has been addressed to the point of exhaustion on this thread is...please read the thread.
But I'll bite.
The human anus (ooh, sorry Gloriia - should I say 'bum hole'?), however carefully cleaned and however scrupulous the individual about personal hygiene, is going to have bacteria around it, as with all animals. It's a fact. It's not a shameful fact - it's biological reality. Even cats, which lick their bottoms clean, will not eliminate all the bacteria there.
I have described above how this can be a problem when an essentially bare backside is placed on a seat. My only wish, in all this, is that people put on a sarong, kaftan, dress, shorts or similar before going to a restaurant for lunch.
I know OP mentioned that some people there (not her, but she's been piled on for it) were staring and even laughing. I'd guess that this might have more to do with the expectations of a little formality and courtesy while dining in France rather than prudery or ogling.
I really, really can't explain it any more. I've tried and tried on this thread....and then a newbie comes along and asks for another explanation. Just as well I'm a bloody fast typist!