I'm glad you have sat him down and had a talk. If he does it again I would seriously want him to get some form of counselling because this behaviour is not appropriate.
Sure you may find your stepmother attractive and there is no harm in thinking. At 13 you are perfectly capable of understanding what you are doing. You aren't a curious 5 year old trying something new, you are a teenager and not stupid. To suggest otherwise is insulting to the majority of 13 year olds.
If the step parent was to do nothing the stepchild may think that this behaviour is OK, because no punishment has been issued. They may not do it again or they may do it again. But if they do it again, it could be to someone at school, or to someone on a train or bus and I can almost guarantee they wont be 'understanding' about it.
You be understanding about a 15 year old trying their first cigarette, you be understanding if your son buys a condom and has a girlfriend and he just wanted to be safe with but it's ridiculous to expect someone to be understanding about having their boundaries breached.
Also, why should she have to put up with being inappropriately touched, it's bloody horrible.
If it was in the manual of step parenting that your stepchildren are allowed to experiment with you when you are drunk/asleep etc and you are supposed to just let it happen, I imagine most step parents would take a hike and rightly so.
Bonsoir is sending a very dangerous message here and I'm not someone who reacts strongly to things at all.