If being proud of homosexuality is perfectly okay, then why not heterosexuality? Obviously, heterosexuals aren't oppressed because of their sexuality (it's not like it's awful to be a gay in the Western world, either), but there is no reason why they shouldn't be proud of it. Although I personally don't see the need to be proud of anything you really didn't accomplish, there's no need to be ashamed of your sexuality, so of course, straight people are allowed to be proud as well.
I know what you're going to say: Straight people have not been oppressed because of their sexuality! That is entierely true. However, that does not mean that straight people cannot be proud of their sexuality. That's like saying a man can't be proud of his gender because his gender has not been oppressed simply because of their gender. By saying straight pride is offensive, you saying that straight people should not be proud of their sexuality i.E being straight is something to not be proud of. But to be honest, there's no need to be PROUD of your sexuality whether gay or straight but I will not judge you for being proud.
If you are the minority. The majority must always be shamed so that most people are unhappy.