I've replied directly to some comments, but addressed others in sections below.
jollytiddlywink wrote:I am reminded of the saying (the provenance of which I cannot be bothered to look up) that if it wasn't for the male orgasm, humanity would have died out long ago. People don't have sex to procreate. People have sex because its really really good fun! If it wasn't, people wouldn't do it.
I agree but I fail to see how that disproves my point that sex is for procreation. I agree if sex were not pleasurable it would not be done. However I think it is pleasurable because sex is necessary for procreation, pleasurable so that humanity does procreation and survive. If it were not as you said, humanity would have died out long ago.
Secondly, if you are going to insist that sex is for making babies, you would be hard-pressed to explain the ever-lower number of babies being born all over the world each year.
Indeed, that is because people use sex for recreation, divorcing it from its full meaning. Worse still in the West there is a very high rate of abortion which adds to the low birth rate.
If you are going to suggest that sex isn't valid without procreation taking place, you might find you've upset not only the gays, but also every couple in which the woman is over the age of 50, or in which either partner is infertile for whatever other reason, or even couples who have decided not to have children, and have taken whatever contraceptive route they opted for to achieve that outcome.
I would firstly like to pull couples who are infertile out of the equation, just briefly. Homosexual sex and contraceptive heterosexual sex are, as you have identified, similar, by virtue that they are closed to procreation; first inherently and the second by deliberate choice. In both instances there is no, or in the latter case very little, chance of procreation. (Although abortion is there as a backup

) Amongst the (non-deliberately) infertile couples, provided they have sex without contraception, that is open, especially psychologically, to the remote potential of procreation, then they are doing so acknowledging the purpose of sex. Their situation can be ameliorated through adoption.
There is no evidence (or at least, no scientifically valid evidence) to suggest that a child needs parents of both genders.
Surely it is mere common sense, children learn by example, they need to understand how to interact with both genders. Freud, not that I'm a huge fan, spent a lot of his research on childhood interactions between the child and its mother and father.
Single Mothers issueI'm not launching a crusade against single mothers, family breakdown happens and it is most regrettable. What I do condemn is planning to raise a child alone. I have said before on other threads, that society has made it seem unacceptable that unless women got back to work quickly after having a child, this is wrong. Childcare is expensive and so is the cost of living, which means often both parents must work. This leads onto a point about the nuclear family by jollytiddlywink, I, by advocating for a mother and father, am merely supporting the idea of the basic unit of a family: a father and a mother plus their children. I agree the nuclear family is a rather new phenomenon and quite regrettable really, for it would provide working mothers with access to free childcare in the form of the extended family, most often in the form of their own mothers. Extended families would make raising children so much easier and cheaper. With extended families there is even more variety of role models from both genders.
Stability of same sex unionsHow stable are same sex unions? Given they are an untried concept, based on the principle that sex is for recreation than why limit it to just one partner? This point applies to contracepting heterosexual couples. The difference in marriage is that there is a commitment between a man and a woman to create a new family, which means children. This requires stability.
A harmonious societyIt is probably not over generalising too much to say that 'broken homes' produce more NEETs (Not in Employment, Education and Training) and young criminals than stable ones. As an aside, I would say that the idea that unless one has gone to University, one has somehow failed in life, is a fallacy and a damaging one for society. It will breed resentment, devalue University education and produce a cohort of young people who are qualified for jobs that do not exist, but that's what you get from socialist governments - lots of things nobody wants.
Same sex attractionJust because there are example of same sex attraction (SSA) amongst other species does not disprove my argument. I was not suggesting that SSA was a choice, acting on SSA is however. The frequency of SSA attraction was mentioned above, it is low enough amongst humanity and other species to understand homosexual acts as a maladaptive behaviour. That is not conducive to the survival of the species or the propagation of that individual's genes. There is no evidence to suggest that SSA is genetically linked, unlike red hair, although I wait to be proven wrong. It is more likely to be a consequence of childhood psychological development.
Abuse of the childAs I said to "[...] wilfully set about this is an abuse against the child". Single mothers / fathers through unavoidable circumstances are regrettable. However to attempt to raise a child single in a premeditated fashion is an abuse. For at very least the child has a right to be brought up by its mother and father. Additionally both are necessary for psychological development and subsequent successful interaction and integration with society.
"Progress should mean that we are always changing the world to fit the vision, instead we are always changing the vision."
- G.K. Chesterton, Orthodoxy, 1908