Bonn-Beuel, West Germany — On the left bank of the Rhine holidaymakers rode bicycles, pedaled boats, and walked their dachshunds. On the right bank young peace demonstrators lofted homemade banners with slogans such as ''to be or NATO be,'' and got tans in the hot sun.
In the middle of the Rhine, Swiss barges, the Romantica excursion boat, and the Moby Dick (with windows designed for some reason or other to look like sharks' teeth) plied the waves. It was your typical combined Corpus Christi and NATO summit day in Bonn June 10.
The antinuclear demonstrators were fewer in number than the 300,000 who captured headlines last October in the biggest demonstration in West German history. Organizers said 250,000 showed up this time. Police declined to estimate the crowd.
The demonstrators were also more subdued this time.
''Last time we sang songs and heard Belafonte'' said one young woman near Tubingen who was dangling her feet in the Rhine. She thought it was livelier then.
Conversations with her and several other protestors suggested three reasons for the more subdued mood eight months after the October exhibition of a growing peace movement here.
The first is discouragement about actually achieving the main goal of preventing deployment of planned NATO nuclear missiles.
The second is the inroads President Reagan has made in recent month.
Illustrating the discouragement, one young father from Trier with a several-months-old child and an anti-Reagan poster noted that he has not given up hopes of blocking deployment of the NATO missiles. But his hopes have certainly ''shrunk.''
The hopes of preventing deployment were equally modest for Susanne Stief-Elhardt, a young woman from Bavaria who was sitting in the shade of an impromptu tent made out of banners reading: ''No nuclear bombs in Neu Ulm and elsewhere'' and ''A trip that is worth it: Mr. Reagan to the moon.''
Her aim in joining the bomb protest was ''simply to work together to keep the peace, to be one person in the mass, to make the mass bigger.''
Her companion, Heinrich Tobisha, still hoped that the demonstration would have an affect ''on the mass of people and on the liberal press so that the discussion and the process of discussion goes ahead'' -- a much more modest aim than once galvanized the peace movement.