I think your logic is good, but your application is lacking. Same-sex "sexual activity" is not what has been outlawed in Singapore. There is no law that fits the description you have outlined and it is essentially a straw-man argument.
The law that exists states explicitly that anal and oral sex, only between members of the same sex, is illegal. Both of these acts have been specifically made legal for a heterosexual pair.
The only difference between these two scenarios? The gender of one participant. A woman may receive anal sex from a man, but a man may not. Discrimination based on gender.
> Same sex sexual activity is not what has been outlawed in Singapore
Besides that I generally approve your comment, let's quote the section that was judged by the Court of Appeal 3 days ago as conform to the constitution:
""" Article 377A
Any male person who, in public or in private, commits, or abets the commission of, or procures or attempts to procure the commission by any male person of, any act of gross indecency with another male person, shall be punished with imprisonment for a term which may extend to 2 years.
The law that exists states explicitly that anal and oral sex, only between members of the same sex, is illegal. Both of these acts have been specifically made legal for a heterosexual pair.
The only difference between these two scenarios? The gender of one participant. A woman may receive anal sex from a man, but a man may not. Discrimination based on gender.