I dont necessarily think that Joker is homosexual per se. i think that with batman he is just fixated, in the same way that gollum is with the one ring. calling batman his sweetie and such are like gollum calling the ring his precious. hes obsessed. I think there are definitely homosexual undertones, but i feel they are done more as a means of showing how creepy the joker can be and that he is defined by batman, just as batman is by him. again, hes just extremely fixated. 

 as well, joker is extremely narcissistic. he loves himself and is in love with himself, just like he is with batman. and his fixation is meant to be shown as something unnatural (which im not arguing that homosexuality is or isnt, just that the jokers obsession is) and something that would get under batmans skin, like a taunt. 

 if that doesnt satisfy you, see the above post. hes nuts.