If they're consenting adults and they're not doing it in public, it's their business, not mine...and that's the way it should be for anything. One's own body is the greatest possession that someone will ever have, and it ought to be their right to use it the way that they want to so long as they're not hurting anyone else. If I don't want to see it, promise not to show it to me and I'll promise not to look for it.
They want to open one up next door to me? More power to them, but keep it indoors, please. (This goes for both the swingers and the Baptists.) Both seem to be pretty harmless activities if you ask me.
Or, Satori's First Rule of Life: as long as you're not hurting anyone but yourself, if you'll agree to tolerate my bad habits, I'll agree to tolerate yours.