Drinking on the Job

Listen up, buttercup. The joke is that the gay man was "drinking on the job" at the sperm bank. But "drinking" has a double meaning: he was either drinking alcohol (bad for a sperm bank employee) or ingesting sperm (because he's gay). It's funny because it uses the ambiguity of the word "drinking" to create a crude scenario. You probably thought he was dehydrated or something, didn't you?