There is a difference (to them) as someone can abstain from sex, you can't abstain from your race.
There are two things wrong with this statement that a JW might make.
1. Gay people aren't simply being asked to abstain from sex the way unmarried heterosexual JWs are. They are told to renounce any possibility of a satisfying intimate relationship on pain of being branded a sinner fit for destruction. This is in no way equal to an unmarried JW who may hope and aspire to an intimate relationship even if it never happens.
2. People modify their race all the time. Unless they conceive of "race" as some fixed genetic category rather than a socially costructed marker of identity. In which case I think we are in different centuries.