Located right alongside the North Sea, gay Amsterdam does have a few beach options within easy reach of its city center. OK, so don't expect Barcelona-style weather or crystal clear tropical water, but what's good enough for the locals is good enough for you!
The best gay beach in Amsterdam is at Zandvoort, which you can reach within one hour of leaving Central Station. Come summer, you'll find most of Amsterdam's gay community packing to the south side of this beach and splashing about. In fact, it's a beautfiful spot to swim and catch some tan.
If you don't want to travel quite as far, another option is Nieuwe Meer, a lake which allows naked sunbathing (and other activities in the nearby woods!).