If same sex marriages are legal, why can’t a man marry two or three women, so long as he can afford them?
Or not afford them? After all, that would be “discrimination”.
Seriously, they often duck the polygamy question since they have no argument against it.
This is the real crux of the problem with re-defining marriage. Once you re-define it for gays, you simply cannot say no to other redefinitions.
If it is okay for two men to be included in the definition of marriage, then (as you say) why not one man and multiple women, or vice versa, one woman and multiple men. You simply have no legal basis on which to say no to any other grouping of people that someone comes up with.
Redfining marriage for "gays" is just the beginning, it will never stop, the redefining will go on and on. Eventually the term marriage is meaninless, and then the people pushing this have won their objective. They want to be rid of marriage, and they are on their way to succeeding.