in what ways will gays having the legal right to marry the person they love affect society?
It will free up all the lawyers who are currently engaged in setting up powers of attorney, living wills, adoption of partner's children, etc., to get some semblance of the legal relationship inherent in marriage, allowing them to spend more of their time on frivolous lawsuits.
will it make it easier for gays to adopt? is that a good thing or a bad thing?
Possibly, and yes, it's a good thing.
will it lead the way towards polygamous and incestuous marriages? can you open one gate so to speak, but not the others?
I think polygamous and incestuous marriages are a very different kettle of fish from same-sex marriages.
In the case of polygamy, the decisions of one person can affect the rights of another: when a person who already has one spouse chooses to marry another, this changes all sorts of things. Take life insurance or estates: without a clear beneficiary/will, everything would go to the spouse; in the case of two or more spouses, the estate and/or insurance payout would be split between the spouses. Should wife #1 get less money just because of an agreement between her husband and wife #2? If the husband was a widower before he married either of them and brought children with him into the relationship, who gets custody? Wife #1 or wife #2?
I suppose a governmental or societal determination on these sorts of things could conceivably be made, but it's enough of a hump in the "slippery slope" that polygamy isn't the logical conclusion of legalizing same-sex marriage.
As for incestuous marriages... I don't know. Assuming you're dealing with adults (i.e. you're not talking about legalizing pedophilia), there's a definite societal imperative to prohibit procreation between close blood relatives, but whether that means that non-procreative incestuous marriages should be legislated against as well, I'm not too sure. My personal opinion is that this is enough of a reason to say "no" to legalization of incestuous marriages.
I don't think there's a lot of hue and cry from the incestuous marriage camp anyhow, so I don't know how big an issue it is.