Lets look at your points one-by-one:

1: It harms the individuals who engage in it by dramatically increasing the chances of untimely death. Only the most advanced medical technologies have had any impact on the natural result of this activity, which even this gay-rights site unwittingly admits. http://www.brucegarrett.com/br​ucelog/789

I read that ridiculously one-sided Holsinger document. You apparently haven’t. Among other things, he condemns various sexual behaviors only when perpetrated by homosexual men – not by heterosexual couples, or homosexual women. Also, the document you cite doesn’t address lesbian couples at all – find me some statistics indicating increased mortality in THAT group, willya’? [Good luck]. Last, but this logic, spelunking, skydiving and volunteering for armed services are immoral behaviors. Please.

2: It harms society because it is inherently selfish. Admittedly, much hetero sex is also selfish in nature, but marraige is designed to reduce or eliminate the selfish nature of this relationship (especially on the part of men) and encourage self-sacrifice and consideration for the needs of another who is dramatically different then you are. Simply engaging in mutual sexual gratification does not contribute to maturity and growth. Learning to respect and appreciate the needs of someone else and meeting those needs even when they are contrary to your own is an essential part of learning to be a contributiting, beneficial member of society.

You’re completely discounting homosexual parents, and the very argument you use can also be used to support gay marriage. You make statements like “simply engaging in mutual sexual gratification” as though this is all that occurs in homosexual relationships. This reveals you to be ignorant, in addition to demonstrably bigoted.

3: It harms children because kids need a mother and a father. No other parental combination can reproduce its benefits.

Yet there is not one iota of evidence that indicates the adjustment of children raised by homosexual couples are any less well-adjusted as those raised by heterosexual couples. You have no evidence.

All this to say, yes, homosexual sex should be legal. But it does not deserve the societal sanction of marraige. Because is doesn't provide the social benefits of marraige.

Homosexual couples are every bit as capable of raising offspring as heterosexual couples. Also, not all heterosexual couples choose to have children. Your argument is incorrect on both sides of the equation.

Oh, one other thing. I've already said that biology plays a significant role in sexual orientation. But that is not the same thing as saying biology determines sexual orientation. We strengthen our innate desires by acting on them, and weaken them by denying them. I weaken my desires for porn and promiscuity by refusing to engage in it, and over time the temptation reduces.

You are also proving yourself ignorant of the most current research on both gender identity and sexual preference. If you’re not basing your opinion on the best available evidence, then what are you basing your opinion on? Also, you do not alter your heterosexual nature by refusing to engage in heterosexual sex. You seem to be implying, however, that homosexual people somehow mitigate their homosexual tendencies by refraining from homosexual sex. By the reverse token, do you think it possible to strengthen "heterosexual tendencies" if homosexuals engage in straight sex? The reverse doesn't appear to be true (as you note below).

You think homosexuality is different? Then riddle me this: Why do upwards of 50% of prison inmates engage in gay sex? And why do they stop again as soon as they are out?

This is perhaps the most ridiculous statement of all. What it demonstrates is the amazing strength of the innate desire to engage in sexual behavior. That inmates stop once their out demonstrates the “apples and oranges” aspect of your argument. What they’re engaging in is a pale substitute for what they desire, in much the same sense that homosexuals often engage in heterosexual sex, but find it less satisfying/preferable to homosexual sex.

You fail to make a case against homosexual marriage. In fact, every one of your arguments is demonstrably fallacious.