Answering Some Arguments Against Marriage Equality

1. Gay marriages destroy the purpose of procreation in marriage, and leads to the destruction of the ‘conjugal’ and ‘fruitful’ reasons for marriage.
If we are to be strict about this, any proven infertile couple should be excluded too. However, that would taint marriage as a harsh and exclusionary institution, and in an age where it has become an option rather than a necessity, would drive many away from it. In the eyes of many particularly younger people, excluding gay couples is just as cruel. Again, rigidity should not apply. We can still strongly stress that marriage is for procreation as a primary purpose, whilst still stressing that we need to be inclusive and compassionate otherwise, and therefore cannot use black-and-white rules to shut people out. This isn’t too hard to understand, is it?

2. Marriage is meant to be a complimentary institution.
This is a useful concept, but a really wishy-washy rule. Not all opposite sex couples are complimentary in their character, and certainly not all opposite sex couples are complimentary in a particular way that same sex couples cannot be, unless you are talking about the physical only.
If physical complementariness is what you're talking about, many infertile couples should be excluded too, as should couples were one member was born with certain 'intersex' medical conditions (e.g. Klinefelter's syndrome - look it up if you don't know what it is), as they are not strictly complementary either. The trouble is that, many men with Klinefelter's don't even know they have it! Again, complementariness is often the case, but we cannot be rigid here.
If spiritual complementariness is what you're talking about, many straight couples come together because of similarity rather than complementariness (think many geeky couples), or because they are complimentary in non-traditional ways (e.g. the alpha female and the omega male), and these have clear parallels in same sex couples too. Of course most people still live out traditional gender roles and always will. But society has already decided that non-traditional gender roles are OK too for the minority whose lives are like that - and as long as they are heterosexual they can legally marry too. Therefore, if those people are allowed to marry legally, why not same sex couples?

3. Gay Couples are Not the Same Thing as Straight Couples
This tends to not be very convincing for those who know gay couples well, again more commonly found in the younger generation. Therefore, this argument is often voiced and accepted by older opponents of equality. And for good reason too - for those who have observed and known gay couples, they will find that there isn't a clear line they can draw, except regarding physical body parts.
Different straight couples bond over different things or reasons, and their relationships are held together by very different central concepts. Technically, they can be divided into different categories too. In this sense maybe we can have the glamour-marriage, the religious-marriage, the Chinese-marriage, the Irish-marriage and so on. But we don't do that - marriage is a broad church and a society wide brand, and for a good reason. On the other hand, one straight couple's relationship may have more in common with a same sex couple's relationship than with another straight couple, other than the body parts. So it doesn't make sense to draw the line at the body parts thing, right?

4. It has Historically Been This Way
There was much confusion and ignorance around same sex attracted people in history. They were just not allowed to live openly as couples. Now that this is no longer the case, not only is denial of legal marriage ridiculous and arbitrary, it threatens to delegitimize marriage in an age where already it is seen as 'only a choice'. In fact, that some couples can be allowed to live openly in relationships but not to be married is entirely the creation of the ‘liberation’ of the 70s and 80s anyway. As conservatives, we should not just allow marriage equality, but we should encourage gay couples to get married, like society encouraged everyone living in a couple relationship to get married bak when family values were strong. Marriage equality presents a chance to re-assert pre-liberation culture.

5. The Family has been destroyed enough in the 20th century
This had nothing to do with gay people, and it was all due to ‘liberation ideology’, which marriage equality does not come from and is often diametrically opposed to. Many marriage equality supporters are opposed to ‘liberation ideology’ in almost every form. It is also inconceivable that marriage equality would lead to any change in most marriages, the way that no fault divorce changed the landscape for example. No fault divorce applied to every single marriage, whilst same sex marriages do not change a thing about any heterosexual marriage.
In fact, for marriage promotion to work and to have a strong case against liberation ideology, we need to show everyone that traditional values are to be aspired for, can be aspired to by everybody, and are not bigoted or apartheid-supporting. When there is a consensus about gay people not being able to change their sexual orientation, if marriage excludes gay people it will always be seen as bigoted and apartheid supporting by a large proportion of the younger population.

6. It Defeats the State’s Purpose of Benefiting Marriage
If we think of the purpose of marriage in society narrowly, the state also should not benefit any infertile and childless marriages. However, part of the way marriage works is by upholding marriage as a brand for the whole society, and that brand needs to be inclusive and non-bigoted to have the most appeal to potential supporters for it to work that way. The benefits of this will return to the majority of marriages – i.e. child bearing, fertile heterosexual marriages, by encouraging marriage and a strong marriage culture in the whole population. Hence same sex marriages still support the state’s purpose of benefitting marriage, although more indirectly. Moreover, further benefits can be seen in a general return to commitment and family values, reduction of STI and HIV rates in certain communities, etc.

7. It Imposes Its Acceptance on All Society
This is a myth. See the Canadian Civil Marriage Act 2005 and what it says, for example. When a law is written that way, to impose its acceptance on all society would require ANOTHER change in law, which may even be unconstitutional in most countries.
On the other hand, opponents of marriage equality are forcing THEIR version of marriage down the throats of everybody else, by having it enshrined in national law. It doesn't matter than theirs is the traditional version - it is no longer accepted as a consensus in most of the West either, and is thought of as offensive by many people in our society.
When there is a clear conflict between two visions of a shared thing (our laws), we should proceed to consider the most inclusive solution. As the equal definition of marriage also includes all heterosexual marriages, but the heterosexual definition excludes same sex marriages, society should opt for the more inclusive definition, which would satisfy to a degree everyone on both sides, as nobody's actual rights get compromised.
Reports about businesses being 'forced' to serve gay couples have not been due to marriage laws. In fact, many such reports have come from Australia, the UK and many other countries where marriage equality is not yet the law. It has to do with the anti-discrimination law in these countries, which often say that businesses must not serve gay people, and by extension gay couples, any differently to the way they serve straight people. Marriage does not appear to factor into this.