If we are going to try and make an argument for anti-homosexual... I'm very pro sex and sex before marriage but let's talk like it's three hundred years ago.Homophobia is a near universal cultural trait, that has persisted throughout history. There must be something to it to have survived so long.
I think we first have to suggest that a lifestyle that chases pleasure and satiating an animal need isn't conducive to a functioning society, which progresses more when people sacrifice their own needs to work at growth for the community.
Heterosexual sex serves a purpose of making more people. So keeping that in marriage would keep the raging heteros from spending their lives engaged in the pursuit of sex, at least that is what I think the theory behind that is from the old school conservatives. That is where I think historical homophobia has come from.
I don't think that's an argument against gay marriage though, because keeping the purely pleasure sex in a union, although not producing more people, isn't a distraction as much as sex with anyone and as a union they would build their lives with a goal that is at least common to them, which I think builds the society by default anyway.
The association of gay sex being purely pleasurable, self serving behaviour is part of why I think HIV was so discounted. One of the most crippling things against HIV management is the association of the virus with a selfish, hedonistic lifestyle. Intravenous drug users, scores of sexual partners meant that unless you contracted the disease from blood transfusion or being unable to carry more than a few condoms without being arrested then you put that needle or man into you. Now, unless you share needles or have lots of unprotected sex, or someone criminally infects you, your chance of contracting HIV is practically zero.
I don't think it makes a huge amount of sense because the animal imperative to get your hump on is in almost everyone, pretending that a man on my business is different to a woman because he might knock me up and it not just being about having a good time is being intentionally blinkered. I'd argue that children from sex has never been a majority motivation in human sex in history.
But that is where I think it stems from. We can have a discussion about whether a society of individuals chasing a fix for a physical need is a detriment to society on the whole, of all people gay and straight, and that isn't restricted to sex either but short of us all pretending that sex isn't going to happen unless it's in a marriage if that were the social law is foolish.
Long story short. I think old school societies saw gay people having fun without the consequences of children being born, didn't like that, decided that those who could more freely have sex would spend more time enjoying themselves than those working to keep the farm growing etc - rightly or wrongly and that's where historical homophobia comes from. In a 'don't eat pigs because they carry disease humans can catch, we don't understand that, let's just say god said not to' sort of way.
I need to clarify here that I'm very pro sex, pro sex before marriage, pro gay and very pro gay sex, I have enjoyed a lot of it. If we are trying to understand historical perspectives we need to explore what would have them create a theory they see as rational and that is what I have done here.