> Straight couples can reproduce with their spouse - gays cannot. That's not just a difference, but a complete opposite.
You're just stating explicitly the implicit argument I was refuting, but ok:
The difference is immaterial for our purposes. The fact that one or both parents in a family do not have a biological link to their children is irrelevant. It's a curiosity of genetics that has little-to-nothing to do with how a family functions on a social level, which is the level marriage laws operate on.
Are we banning adoptions and step-children now? Society has no reason to outlaw couples that can't reproduce together. Homosexuals can adopt, they can use surrogates, they can have children from previous marriages (ended in divorce, or death, etc). As long as we're think-of-the-children-ing it would be better all around to let those children grow up in families if possible.
More importantly, that's not a difference between heterosexual and homosexual couples - it's a difference between couples. Many straight people can not or will not reproduce with their spouse either - are we going to ban marriages between infertile people? Force people to have children?
The point is this: if you pick at random a married gay couple...and then pick at random a married straight couple....the difference between them will be no greater than the difference that we already tolerate between heterosexual couples. Therefore, you cannot argue that we must ban gay marriage on the basis of preventing some problematic aspect of gay marriage - any "problem" gay marriage is one heterosexual marriage also has. It's a largely artificial distinction. If not being able to have children together is SUCH a big deal, then argue against that and let gay couples be banned under that banner.
If you're going to ban it on the basis that a gay couple is statistically much more likely to have quality X, then quality X must be a VERY serious thing. Infertility is not serious enough to qualify.
Note too this situation may change. The science of reproduction is ever evolving and changing.
> These are good reasons in my opinion.
> is that I don't want the definition to change
This is either silly and arbitrary, or begging the question.
You don't want it to change why? Because you believe laws should never change? You're afraid of all the paper we'll waste printing out new forms? You don't want to have to memorize new legislation?
No, let's be honest - you don't want it to change because you don't agree with the proposed change. That's fine...but just state that instead of coming up with some silly dodge like "I don't want the definition to change".
> I don't want my children having gay propaganda pushed on them
Here I suspect is your most honest; you are against it because you don't like homosexuality. Fine, that's your right to feel that way. However you should just be honest with yourself and others and admit that, instead of providing silly and flimsy rationalizations which fall apart under any scrutiny.