It's about the word marriage. For me it is defined as a bond between a man and a woman

Then I explained what the church wanted to achive with a marriage. People that will raise their children together. And back in the day, usually a male and a female had a child. It wasn't so common for a gay couple to raise a child I guess.
@the great statistic:
I'm usually skeptic towards statistics generally XD Since you can easily manipulate it when you leave something out.
But what the educationalist did was this (I got that from my prof at uni. So most likely true

):
They visited families (a lot of them, at least 100) and watched the family. They counted the time that a parent would interact with his/her child (in minutes or whatever). So if they talk with them, eat together with them, play with them, read them something etc.
It turned out, that parents spend more time with their genetic children than with adopted or sick children. The parents themselves don't notice that tho, since they were asked before what they think. If they spend more time with their own child or not or the same time. Every parent said they threat their child equally.
So, what the statistic said about a "better parent" was, if they spend more time on their child or not. Of course it doesn't tell how they interact with their children

But this is a way to check it out without being too subjective.
There was another test with families with a adopted child and a genetic child in one family. They tested the children on a stress hormone. The adopted child produced a lot more than the other child. A reason that they told us was, that the adopted child fears to be kicked out, thus is trying harder/has stress. But the child said he/she is felling good in that family too D:
So, why are they teaching us this? :x
Well, as a future teacher, there can always be children that are depressive or have problems. So, you need to know what could be the cause. So, one cause can be the family circumstances.
But like I said. It's statistics and I also said any other parent, if gay or single etc. can be a better parent than a married couple.
But it's always easier if you have a partner, gems, gold and a swimming pool XD