Well, I'll commend you for not bringing religion into this, so I'll try my best to respect your religion and yaddah yaddah "I hate everyone equally" etc. So I'll just defend against your points one by one.
1. While yes, animals' purposes in nature are to reproduce, there are commonly reoccuring defects (if you wish to call them that). And just because an animal has homosexual tendencies doesn't mean that they won't mate. Think of it like, "I'll have to mate to give birth, but other than that I can do whatever." From the BBC, we have this article: http://www.bbc.com/earth/story/20150206-are-there-any-homosexual-animals, which claims the same thing I do. While humans are the only "true" homosexual animals (as in the fact that some homosexuals choose not to breed for reasons that can't be explained by selective breeding or mating outside of one's preferences). The closest would be domestic sheep, in that the mutation that causes females to desire more sexual encounters (a smaller hypothalamus) also has an effect on males that induces homosexual desires. So as I've said, it is possible for homosexual animals to exist within natural selection. Because the increased fertility within females offsets the approximate 8% of males that won't mate. The findings correlate with Simon LeVay's studies on the differences between gay and straight mens' brains: http://science.sciencemag.org/content/253/5023/1034, essentially saying that gay mens' hypothalamus is approximately the same size as females'.
2. Well I can see why you wouldn't want a 50 year old marrying a 6 year old, despite the fact that approximately 207,000 children have been married in the past 15 years (averaging to be 13,400 children per year). In most states, it's legal for a child of any age to be married with parental consent (so some creep with the hots for his daughter could abuse her until she gives her consent to the officials, which might not even be necessary if she's an unknowing girl and/or she's afraid of her father). Evidence here: http://theconversation.com/child-marriage-is-still-legal-in-the-us-88846, but that's off topic. While of course your religion and culture prohibits same sex marriage, you do have to realize that other cultures that marry have different beliefs. Take the Romans, "At least two of the Roman Emperors were in same-sex unions; and in fact, thirteen out of the first fourteen Roman Emperors are held to have been bisexual or exclusively homosexual. The first Roman emperor to have married a man was Nero, who is reported to have married two other men on different occasions." Link here: https://en.wikipedia.org/wiki/History_of_same-sex_unions, but of course same sex marriage was prohibited by later Christian emperors. Not to mention the Native Americans, "In North America, among the Native Americans societies, same-sex unions have taken place with persons known as Two-Spirit types. These are individuals who fulfill one of many mixed gender roles in First Nations and Native American tribes. "In many tribes, individuals who entered into same-sex relationships were considered holy and treated with utmost respect and acceptance," according to anthropologist Brian Gilley" (same link). So overall, you can't restrict marriage to those who aren't part of your religion/culture simply because yours disagrees with it. Marriage isn't a strictly Christian idea, so to say.
3. I see where you're coming from, but I would have to disagree. There's this thing called...the internet. If you're a lesbian couple raising a boy, and he needs to know how to shave, bam there's a WikiHow article on it: https://www.wikihow.com/Shave, need to learn about periods for a gay couple, bam there are SO many articles about sexual health and reproductive organs. It's called picking up a book and/or Google. And if your concern roots from a more basic idea that men and women are inherently different psychologically, well I can't really dispute you. I'm not a big fan of gender politics and/or sciences, so I can't really back up either side. What I can say though is that in most homosexual relationships, one side takes over the more masculine/feminine side. No matter which relationship you're in, there's a top and there's a bottom, so to say. There's the one that goes to work, and there's the one that cooks (if you care for a more outdated approach). And if you're still worried about reproduction, there's a host of children that are just waiting to be adopted. Not to mention that there's this place called a Sperm Bank for lesbian parents, and these people called surrogate mothers for gay parents. Of course, that's assuming you want to impregnate/be impregnated. Most gay parents I've met simply adopted, which of course helps not have a bunch of kids on the street without parents. And I also can't really argue about the immigration thing because I've purposely distanced myself from immigration arguments, simply because it either turns incredibly racist or incredibly "let's just take EVERYONE in," both of which are kinda (incredibly) stupid.
Hope this was good enough for you. xD