My explanation for it is Brian really does love Justin but he rejects the whole prescribed, romance cliches like buying roses, picnics on the floor, saying I love you, etc. Being against society's definition of romance and monogamy is a part of who he is so when people bring it up to try to make him do it, he rejects it and goes in the opposite direction. To Brian, being his own person and following his values is more important than trying to make other people happy even if it makes him cold when he really is loving (which he shows more by kissing/sex-see 219 end). Also in regard to the picnic, he had a bad day at work and just wanted to relax which in his world his dancing and fucking at Babylon.