Everyone doesn't enjoy equal rights. Adulterers, fornicators, polygamists, the incestuous, molesters, just to name a few, are discriminated against. They don't enjoy equal rights. What makes homosexual activity so special? What exactly is contributed to society at large by such sexual activity? NOTHING.
Adulterers break a legal agreement (or at least I think that marriage is normally considered a legal agreement). Fornicators, depending, do enjoy legal rights. They can fornicate, and then get married. Polygamist are mostly kept out by irrational prejudice (or is that redundant), by the fact there are that many who are open about it, and by the fact it is extremely complicated to revamp the legal system to accommodate them. This does not mean they shouldn't have their rights, just that they aren't given. Incestuous are a result of the first two issues that the polygamist have combined with a major misunderstanding of genetics by the population with another little dab of irrationality. Finally, molesters do not get equal rights because they are harming another. We may have slight disagreements on what constitutes molestation, but molestation is harmful to another, and so they do not get 'equal rights', even though the Bible does specify that some molesters should (read Duet. if you want to see for yourself).
And do you know something special, current heterosexual activity does not contribute to society on average. There is a very general trend of lower IQ equal more offspring. So actually, it goes likes this.
High IQ heterosexual sex with chance of reproduction helps society.
Any IQ homosexual sex/ any heterosexual sex without chance of reproduction does nothing good or bad for society.
Low IQ heterosexual sex with chance of reproduction tends to reproduce people with lower IQ's on average, which is harmful to society COMPARED TO the other options.
Of course, IQ is not the best scale to use, as there are benefits to society to be offered from certain individuals which does not show up on an IQ test, which is why we should looking into MI test, but even then, the general trend still holds. The smarter you are, the more likely you will control your reproduction, and since 'smartness' is in a large part genetic, it results in more of the population being made up of those not as smart. Of course, one needs to handle this issue without resulting in elitism, but how to handle this is completely off topic to this thread.