Nevermind how long it has been going on for, or how it occurs in all species known to man, you still can't call a characteristic unique to 4%-8% of the population "normal".
First, you absolutely can, if it occurs normally.
Firstly, you realise that you;re defining a word using the word itself? Secondly, outside of specialist usage (in statistics, measurements, sciences, etc) normal means "common", "not rare", etc. I suspect you mean that something is normal if it occurs naturally, in which case you would then have to concede that serial killers are normal as they occur naturally, and planet-destroying asteroids are normal 'cos *they* occur naturally too, and sociopaths are normal because they occur naturally too. This is a (psycho)path you don't want to go down :-)
Second, we don't know what percentage of the population if would normally occur in, absent the anti-homosexual propaganda. I suspect a lot more people would identify as bisexual, given a chance. And we know that when there is less repression, more people identify as homosexual.
Regardless of what you *suspect*, what we *currently know* leads us to believe that homosexual relationships are most certainly not normal.You can argue that they are natural, that the characteristic does not diminish the individuals worth, that they are worth having in society, that that homosexual individuals deserve to be treated the same as the rest of the population, and I'd agree with all of that. You can argue that they should have all the benefits (or lack thereof) of marriage and I'm right behind you (not in *that* way you dirty pervert :-)).
However it's hard to agree that a characteristic that occurs rarely is normal, nevermind what the characteristic is. It occurs *rarely*. That means it is not common at all.
The low numbers of homosexual individuals make it, at the very worst, "unusual".
It's usual for some percentage of the population to be homosexual. That percentage is both significant and not apparently diminishing.
Homosexuals are part of the normal sexual spectrum, which has been with us since time immemorial. It's not like it's something that cropped up recently, or only appears now and then.
You seem to be twisting your own words to make the word "normal" appear in your definition. Homosexuals are part of the sexual spectrum, period. Adding the qualifier "normal" to that sentence makes no sense - what the hell is an abnormal/not normal sexual spectrum? The only reason to specify a "normal" sexual spectrum is to push the word "normal" in there somewhere. It's not needed - the sentence makes (more) sense without the "normal" qualifier.
Homosexuals *are* part of the sexual spectrum, but they occur on that spectrum relatively infrequently. That makes them not normal. Homosexuality is not the norm. It is rare.