Hehe, alright I'm sorry its really not funny, but my bitches hump each other constantly and once I figured out what was going on... yeah. This is very difficult to stop, but if you don't want to use your un-neutered male for breeding it may be possible. If you want to use him for breeding you cannot punish him for humping things... then when you want him to, he won't do it (a lot of times anyways).
One thing you should know is that they are masturbating. A lot of people think that dogs hump for dominance reasons, this is generally not so, it feels good so they do it.
Definitely neuter your one dog, that may help decrease the behavior. Otherwise you need to punish/redirect the behavior (I say both her, because both CAN work). Basically if you see them doing it, don't make a big deal out of it (any attn is good attn to some dogs) but immediately ask them to do something else. And praise them for doing the other behavior. Keep on top of it and you may be able to teach them to not do it... at least around you.
Or just ignore it, they may have learned that you freak out when they do it so they get attn, so they keep doing it.
Our bitches still do it and they used to double team our male.... sad I know
but he didn't seem to mind. We just ignored it, figured that its not unnatural and there isn't a reason they shouldn't be doing it as it doesn't hurt anyone. But its weird when people come over!