sure, rome is a outlier, same goes for greece and some other places, but we took most of our culture from those places, the western world stands in a line with rome and ancient greece.  do we read books from some african tribes in school? no we read plato and the De Bello Gallico.  not every culture has the same development level, and we should copy the highly developed cultures.

No, the example from ancient Greece is commonly misunderstood. The relationship between a man and a boy was not viewed in the same lense as that between a man and a woman. These so called pederastic relationships were also temporary and ended when the boy turned 17. Thus, not equitable to modern day same-sex marriage. 

Doesn't matter if it's an outlier, both it and Mesopotomia disprove your false claim that every culture endorsed only heterosexual unions.

Also, do you propose banning marriage for the infertile and those who don't want kids? Because that's the logical extension of your precreation-centric view of marriage.

Marriage is legal recognition of two people's relationship. Procreation and religion aren't necessarily involved at all.

I am married to my Japanese wife, in Japan we are legally recognized as being married, however as I had no intention on moving back to my home country, I made no effort to report the marriage to my native country.

As such, my marriage is legally recognized in Japan, but not legally recognized in my native country.

We're still married, a government, country, institution or establishment not recognizing a couples union as marriage has no bearing on the legetimcy of said marriage beyond legal or political constraints.

This thread is actually pointless. The title should read "I want to get into an argument about something that's not relevant to me"

they didnt had a social system or normal marriage like ours too. but they didint hate on gays or discrimante against them.

You have that backwards. The "just cuz" reasoning is more applicable to gay rights because there is still no strong epistemological reasoning behind it. It's a leap of faith that is politically coloured.  

Marriage is not a right. Point me to a credible source that speaks in favor of marriage as an unqualified and absolute equitable right. It's a privilege that privileges two individuals, historically a man and a woman, against those outside the union. Example, the sexual monopoly of both. An outsider having no sexual claims to either the man or the wife. That's a privilege, not a right. 

how is that union superior? your answer probably is "it can create offspring"

but what about those heterosexual couples that never have babies (intentional or unintentional), why should those be able to marry from your point of view?

additionally earth is actually overpopulated with humans, that is a fact, so I'd rather have us produce less offspring (until we reach a sustainable population size of 4-5 billion) to ensure the continued well being of mankind and earth's other species

Add "talk about something I've already made up my mind about and will refuse to change on." too.

Sperm banks and surrogate mothers make that superiority largely redundant, too.

He's made it clear in previous threads why he is really against marriage equality, he's just using a polite smokescreen now because he was banned for hate speech last time.