By keeping marriage about biological children of both partners, at lest partially, it is perfectly reasonable to exclude gays without being prejudiced whatsoever.
Obviously there is no necessity to change the definition away from this as most people would agree with this.
To be fair you haven't given an actual definition. By making marriage about biological children of both partners, at least partially is exceedingly vague. What does the "at least partially" part refer to? Why can't homosexuals fit into that portion? Why would barren, and elderly couples fit in there?
Making marriage merely based on emotion, makes it a senseless law or, in other words, bad jurisprudence and a waste of resources.
To be fair, sometimes marriage is based on excessive alcohol, fear of being alone, financial security, or even revenge. How do you plan on stopping that?
If instead, it is to protect children from broken homes and society from these children (an argument understandable to the current culture) it is sensible, becomes about heterosexual sex, and highlights the difference, which is children.
We have to protect society from what? Why? How? Explain why marriage is solely about heterosexual sex?
Commonly: Celibacy is healthy, and apparently so is married sex.
Celibacy is not always healthy. We do have a desire for sex, and an attitude of denial is not particularly healthy either. For an example, let's look at Catholic priests. Sex in marriage is healthy, but no more so than a couple who is unwed and have a sex life. In fact, it is all to common to have a deteriorated sex life in marriage. Celibacy until marriage can be disasterous if the couple happens to be sexually incompatible. Trust me, I've seen it happen.
It appears that this is inconsistent with gay sex.
I believe your premise to be flawed, so I don't reach the same conclusion. But why can't two men or women be in love with each other and not be celibate?
If you need to have a discussion about sources, I'll just say that you can find them if you look, even if you don't like the people who wrote the one you find.
This is not a hopeful statement. But, sure I will find some literature as to why celibacy is good for some people. However, there is a ton of literature on why celibacy just isn't healthy for everyone, if you look.
People have various definition, I'm pointing out that logical definition exist that would exclude gays without being discriminatory.
Well, not really. You're argument is based on the assertion that marriage is to benefit children, and society. I'm still waiting for an explanation for how this would be so.
Secondly, you keep referring to the lowest common denominator. Unfortunately, this is how the law works. Because marriage covers people who cannot bear children, or who do not want children without disqualification, there is no reason to disqualify homosexuals. There is no passage in the law, or by the definition of marriage that says: "a union between two people who intend to have children."
Thirdly, this all sort of ignores the role of adoption. If the purpose of such a union is to have children, than adoption would really have to be expelled. Afterall, by your logic infertile parents should not be getting married in the first place, and adopting would be a public admission of their bucking the system, or getting married for emotional reasons. Otherwise, we would have to admit that homosexuals should be allowed to marry because they could adopt children, and still complete the "marital unit."
Finally, explain why our society would use this defintion of marriage (which has not been suitably defined to my taste), except to deny marriage to homosexuals? Redefining marriage in such a way seems prejudicial.