-What about the cases of parents making sex with their own children/family members?
That is incest, not homosexuality. If both parties were adult and consenting and aware of any genetical risks beared by their eventual children, I would say: let them!
-What about the cases of people who like to thief others?
Theft is illegal and has nothing whatsoever to do with homosexuality.
-What about the cases of people who like to not only take drugs but spread them also, one way or another?
If the drug is considered illegal by your legal system, then it is illegal for them to do so. It should be tolerated if the harm they do does not exceed the positive effects (consider the different legal status of smoke, alcohol, weed, coffee and cocaine).
-What about the cases of people who carry guns like it's all over Western Cowboy films again?
Statistics about countries where it is legal to do so (e.g. the USA) compared with statstics of countries where it is not legal to do so would tell us more.
-What about the cases of people who enjoy group sex?
Yeah, what about them? Is it illegal to have group sex or to masturbate?
-What about the cases of people who constantly bully other people around?
They should be disciplined, as their conduct, while not techically illegal, is morally reprehensible.
-What about the cases of people who take money in order to sell out themselves and other people/companies/whatever?
I'm not entirely sure what you are referring to here ...
-What about the cases of people who cheat on their families sexually and in other ways?
Such a behaviour is certainly morally reprehensible and might even be literally illegal in some countries. I would argue that the problem is not the cheating in itself, but the situation underlying it: what is wrong in the family to make the wife or husband go to another for sexual or emotional satisfaction? The cheating in itself is normally not the cause but a consequence of preceding problems: no more love, economical problems, personality crisis etc. Mostly these happen in cases where the partners didn't really thin kthrough their relation. Moreover, when in an early stage, before wedding and children, they cause very little harm.
-What about the cases of people who "make" sex to animals?
If they actually like it ... and the animal is treated appropriately ... well, I'd say let them! Seriously, I am refraining from calling this "unnatural". Mankind has done many and extremely unnatural things in its evolution. One drive has always been the maximalisation of pleasure and the minimalisation of pain. If zoofilia (sex with animals) gives pleasure, well, let them be.
-What about the cases of people who enjoy being sexually abused or abuse others?
I would say that the same applies as in the case of incest: adult, consenting parties with knowledge of the risks involved. I suppose that society could impose some limit on the injuries inflicted at that point beyond which the normal everyday functioning of the succubus/a would be seriously impaired. Moreover, most kind of abuse is moral abuse, as in debasement etc. Phisical sadomasochism taken to the level of physical injury is a pleasure share by a very small minority.
-What about this and that?
I don't know about this and that, but the point in question is why society would prohibit the homosexual couples to obtain the same civil status as heterosexual couples. You didn't really provide reasons for you earlier point ("it's not natural") but gave examples of other behaviour considered unacceptable. I think those examples are wrongly chosen: in almost all of them there are physical risks (genetically weak offspring, physical and mental injury though drug abuse, higher risk of violent death through bearing of firearms,etc.). In the case of homosexual civil marriage none of these apply: homosexual couples already exist, they do not harm each other but love each other, they do not damage society, but simply care or each other, have friends, go to school, work, etc. What is the problem? Just regulate an already existing status quo by providing the relevant laws. We all pursue happiness and one of the limits we encounter in that pursuit are the rights of our fellow humans. Why do we want to stand in the way of homosexual happiness? Move out of the way!
You said it would not be natural: well, is wearing clothes natural? living in cities? would you want us back naked up the trees? Is monogamy natural? What is "natural"? Humans aren't natural at all: we shape nature according to our wishes, even our own nature. We have evolved customs and pleasures which make us far from natural. Where would you draw a line? Would you relly go so far as to say that some people are not human? We have done that in the past: negroes, jews, etc. have been severely discriminated. What if your doughter/son ahd come home with a black or a jewish person there and then? Would you have shunned her/him? WE have accepted those unions in the end because we realised it was the right thing to do: no barrier should exist. We now face the next hurdle: homosexual marriage. i see no reason to prohibit it. Do you?
There has been talk about children: what about children? Homosexual coupes cannot have children by conventional means. Other means exist. What would be resons to deny them to make use of those means? Do we fear the children would be educated badly, wrongly, grow up as perverts? Children from heterosexual couples already do so, but it is mostly children from "broken families" who do so: orphans, children whose parents neglect them, fomr parents who break up, who beat each other and their children, who have a drinking or drug problem, etc. On average, gay couples have thought longer and harder about living together and wouldn't do so if they were not really determined and sure and in love, seeing the opposition they commonly face. I would think that they would provide a loving and stable environment for children and I do not see any reason to think their children will grow up any more wrongly that children of so-called regular couples.
If you think teasin is tha main problem, then you are giving a very bad reason to prohibit gay unions and adoption rights, as the teasing and shunning is done by very nearsighted, closeminded people. THEY are the problem, not homosexuals.