The show has the very good luck to have Garrison and Allen as the mature couple. Shawn Fagan is an easy actor to watch and listen to when his Billy makes love to Rebecca Brooksher’s Maggie. She makes Margaret so very appealing that every man in the audience sighs when she seems about to lose her lover due to an affair that threatens their romance. Daniel J. Kotlowitz’s lighting design keeps the show moving from place to place and time to time. “Lovesong” plays at the Unicorn Theatre on Route 7 in Stockbridge through August 29.