The Taxi agency's ad for the Vancouver Aquarium reminds us that male seahorses gestate their young. A pregnancy test embedded in the bottom of urinals returns a (presumably false) positive result whenever a man uses it.
Similar hormones stimulate the production of sperm in men as those that women produce during pregnancy (might even be exactly the same, this stuff is rusty.) In any case, it is very possible for men to pee on regular old home pregnancy tests and get a positive.