First of all - pro or amdram?
If Amdram:
Pull the crew members asside, separatly. Tell them that whilst you appreciate that they are not getting paid to be here, they are detracting from the show, and it is totally unacceptable. If you have a ready supply of people able to step up and take over their job, be frank and say "Look, if you can't stay focused on the job, I will be forced to find a replacement. Everyone else here is giving up their time to be here and make this show happen, giving a hundred and ten percent. And if you are not willing to do the same, and keep your private life out of the show, it is unfair to everyone else, and I can easily find someone to replace you. I hate doing this, yadda yadda yadda" (read: Lay on the guilt as thickly as you can).
If Professional, things become a lot simpler:
"Can I speak with you for a moment, x and y? I have noticed that your attention is not completely on the show. Whilst I appreciate the fact that you two are involved in a budding relationship, this is not the time or place to explore it. If you continue to ignore your responsibility to the show, I will have no excuse but to give the both of you written warning and to take this up with management. I dont like being a spoil sport, but you are being paid to be here, and whilst you are here and under my supervision, I expect your complete attention to be on the job at hand. I do not like being put into this position, but I have a responsibility to the show and the rest of the cast, crew and creative team to ensure that what the audience sees is the best it can be. So if you could keep your relationship to outside the show, I would be extreamly greatful."