Katie - I do not know what you had Cubby on before and what side effects it caused...
My DIL has her kitty Pickles on Cyproheptadine, a medication that is an antihistimine used for humans - but in cats, it lowers their testosterone. She has not noticed any side effects other than Pickles stopped being aggressive towards his sister and quit spraying. I know that she was VERY worried about using a medication and about side effects so this might be one you could ask the vet about? She also uses the lowest dose and could have raised it but did not need to. Just a thought!!
It sounds like Cubby might have the ...uh....urge? A little reduction in hormone might do the trick. A humping kitty doesn't sound like much fun!