Janet Smith explains it well.
I won't do it justice- but from an NFP standpoint, all I can say is: if a homosexual (with or without AIDS) is having anal sex, isn't he already in grave sin? For that matter, is the condom even contraception?
If he had AIDS, what would make it worse would be infecting his partner. This would be a grave evil.
Another good explanation is here (my article)
and commentary from Lisa Graas.
And Jimmy Akin.
And Fr. Z.