Some folks would say this is a dog acting "jealous". And although this is sometimes the case, I tend to think most of the time the dog is just picking up on the happy, interactive energy and wants to join in the fun.
I have one friend who uses a baby-gate at the doorway, so their dogs can still be near, but out of the middle of the "action".
If a dog gets aggressive towards one of the partners when a couple is showing physical affection, this can be a competitive issue on the part of the pup, or the dog thinking their "favorite" leader is being attacked. Some dogs are also of a slightly more dominant personality, and tend to want to "supervise" the actions of others around them.
Pay attention to what your dog's behavior and body language is telling you. This can help you figure out where this is coming from and how serious it might be.