Swinging is not illegal ... period. Their sex life may not be the focus but their lifestyle will probably be debated ... rightly so I think.
Do you think that having strangers over to your house for the sole purpose of having sex is healthy for adolescent children in the house? Personally I think not. While many who swing may be well adjusted I suspect there is a higher percentage of those who are not sexually well adjusted and need deviant sexual stimulation to satisfy themselves. If the parents want to swing ... fine. Do it before they have children, or after the children grow up and leave home ... or do it off-site after getting a babysitter. Not in a garage with so many people hanging around you find open doors so people can throw cigarette ashes out a "non-event". Again, not illegal, but not a good environment so close to children.