Sri Lanka is very small. The best plan would be to stay a few nights in Negombo, get your guesthouse (Dickmans or Gomez Place) to organise a car tour with a gay friendly guide. This is not expensive - $60-80 a day. Then check the internet for gay guesthouses in Colombo. Cruising on the beach and Galle Face is easy. As earlier posts say discretion is still required. My main advice is to stay at gay guesthouses not big chain hotels where you may have difficulty bring boys back.