I'm straight (and divorced), and we got married for many of the same reasons that LGBT folks want to--it makes sense. Visitation rights, inheritance, tax treatment, etc.
It's unlikely that marriage would be removed as something of concern to lawmakers--has that kind of thing ever happened anywhere, any time, any way?
I respect OP's opinion, but it's ivory tower and we're talking about real people right now.
possibly unnecessary edit: just want to reiterate that I think OP is _right_, it's just not practical, and I'll sacrifice rightness for a rather-large "quick and dirty win".