So it comes down to being prepared and -
Is there a polite way to tell people (family) exactly how wrong it is that they're; a) implying that a parent involving their child in an activity will cause the child to become 'you know'; and b) acting as if a child being 'you know' is a negative, or a threat that should try to be avoided.
OR
Should I just stick to the 'Hmmm, how interesting of you to think that way' and move on. Which I know is easier but I kind of want people (family) to realize what it is they're actually saying and, even if they really feel that way, that I disagree with them.
This is funny. My personal experiences have obviously colored my response, but my first impression when I read your boss' comment was that it translated to: "Aren't you worried that
people might think he's g
ay/feminine?" The "and make fun of him/give him a hard time" is implicit. It would not even occur to me that someone would suggest that a childhood recreational activity would have any bearing whatsoever on someone's sexual orientation. (To me, it makes about as much sense as someone seriously suggesting that the moon is made of cheese or that babies come from the cabbage patch.)
I bring this up not because I think that it's okay to make that sort of comment, but because I think this may be less that they're worried about your son's sexuality and more worried about the fact that society can be cruel. For those reasons, I'd favor Redsoil's response over one that immediately assumes that the speaker buys into the silly stereotype. Even if they do, your cheerful explanation that society is moving on from those ridiculous beliefs will get your message across quite nicely.
It's certainly silly to think that an activity will cause someone to have a particular sexual orientation. It's also sad that some people see particular sexual orientations as something to avoid. However, you'd probably agree that having a child who is routinely bullied would be unfortunate. Unless your family, friends, and co-workers routinely make other types of homophobic remarks, I'd treat this less as "What if he turns out g
ay?" and more as a "What if the other kids make fun of him?" I think that especially if these people are folks that you see frequently, you should try to assume the best.
However, if these folks routinely make other types of homophobic remarks (or they directly come out and ask if you're worried that dancing will make him "that way"), then I vote for calling them out. For comments in the former vein (dance classes will magically determine sexuality), I vote for amused disbelief. "Wait, you don't
actually believe that childhood activities might have an impact on my son's sexual orientation, do you?" If it's more in the second vein (it would be bad if your son was g
ay), I vote for disbelief with a heap of ice: "I hope you don't think so little of me that you believe my love for my children would be dependent upon who they fall in love with."
I hope he does enjoy the classes! Ballet never really appealed to me. I was more of a slime and GI Joes kind of girl.