I have a humper too but she humps my older pug. Both are female and spayed. This leads me to believe it is a dominance issue. Aside from the usual "no" and physically removing her, I've had no success! Even with two bad hind legs...she persists! I'm only thankful that my older girl has the patience of Jobe! Perhaps your fiancé could be helpful and be gentle but stern to dictate that he will not be dominated? I'm certain that there are more knowledgeable people that will chime in. This too, shall pass...
Dominance? Nah, what on earth are they trying to get access to? Dominance is simply primary access to resources, it really can't play a role here. Dominance really does not mean "your dog wants to take over the world". That's probably one of the biggest misunderstandings in dog world, one we really don't need. Your dog is really, really not out to get you.
Teach them something else, like 'sit'. Make sitting really, really worth their while - use their favourite treat and only use that treat for sitting. Then, everytime it looks like they're going for a hump, preferably before they mount, but during will work too, ask them to sit, and reward them quickly and well.
Then, the hard part. Keep it up. Keep it up every time, for however long you need to. One day, you'll see them getting that "I wanna hump" look in their eye, and they'll sit down on their own. That's when you've won, and the urge to hump has now become the urge to sit. Keep rewarding it well, but at that point, you can start switching to occasional, random rewards. Reward every time, but skip time 11. Then skip time 7. Then skip time 13. Then skip time 5. Keep this up, slowly slowly decreasing the amount of time between skipping rewards until you're only rewarding occasionally. This is what eventually makes it permanent.
You can also just distract them with a favourite toy every time they start humping. If they have a favourite toy this may well be the easiest route, since humping like this is often a kind of play (sometimes it's sexual, sometimes it's play, sometimes it's both). The process is the same, when humping looks to be starting, play in stead. Keep it up until they bring you the toy in stead of leg humping.