Translation:You are singing because you are cheerful.
Checking the English wiktionary for "gay", we find that such usage is "dated". Checking further what exactly they mean by 'dated', we see: "Formerly in common use, and still in occasional use, but now unfashionable; for example, wireless in the sense of "broadcast radio tuner", groovy, and gay in the sense of "bright" or "happy" are all dated. Dated is not as strong as archaic or obsolete."
So... why use it? I know this usage of course, but I really think it could be interpreted as "You sing because you are homosexual, singing is gay and not for real men"...
Śpiewacie, bo jesteście weseli - You are singing because you are happy/ merry/ gay
The word happy describes temporary state of the mood and it is commonly used in the meaning "zadowolony", "radosny", "uśmiechnięty", "wesoły", "zawiany", "rozbawiony", and sometimes in the philosophical meaning corresponding to the word "szczęśliwy".
The word cheerful, which means "pogodny", is not appropriate for the above context, because it rather describes a life-long personality trait, general nature or disposition.
All the native speakers I asked do not agree that "cheerful" is a 'life-long personality trait', we also got some comments like "I certainly get cheerful after a nice glass of wine".
We accept "pogodny" here, but we don't agree that it's the best translation of "cheerful". I also frankly don't recall when was the last time I even heard this word. I sure know I hear "cheerful" from time to time.
I'd agree that "happy" is a good translation, but it also overlaps "szczęśliwy" and we use it as the main translation of that word. It's accepted here, of course.
P.S. Wasn't the last time when it was normal to use "gay" in this meaning sometime in the 1950s?
'Bo' is a conjunction and has therefore no case requirements. You can look up on wiktionary which form 'weseli' is.