I have generally had great experiences with boyfriends/husbands showing up for shoots. I always encourage my models to bring their significant others and often they get involved in the action.
Sometimes they like to just sit back and watch the action and other times they want to be involved, which I encourage.
I had one guy who longed to lick and suck on his girl's toes, which drove her insane, and my shoot provided him the opportunity to do so. I'd always like to think they went straight home and fucked.
I've had many boyfriends eager to help tickle my model. It seemes to add a level of excitement, with the model yelling "You'll get yours when we get home."
I had one bad experience. I had a girl who was doing it strictly for the cash and we went through a long song and dance routine to get the husband comfortable. He eventually was pacified, but only because of their need for money. He left after the first few minutes of the video. We made a great video and I wanted to do a whole series with her. The next day I got an email from her saying that her Husband was entirely uncool with the shoot and it threatened her marriage.
How can you prepare for such scenarios?
I have much more to say on this topic, but not just yet.