No, I don't think marriage will be devalued if gay people are allowed to marry - why would it? If two people love each other and want to make a legal committment and public declaration then that's lovely, no matter what their gender. And apart from all the other difficulties in the way of gay families, doesn't the unmarried status of the parents cause problems for the children in inheritance terms? We kids of marriages can inherit something like £250,000 tax-free AND have automatic rights if our parents die without making a will. The child of a gay couple will have no rights over the estate of the non-biological parent which strikes me as desperately unfair.
I'm not sure divorce is easy, tbh. Having seen some close up. But if it was made more difficult that would be very dangerous for women in particular stuck in violent or abusive relationships. I don't think we want to go back to the days when women were automatically dependent on men and second-class citizens. There's enough gender inequality still around without re-introducing some of the stuff we've managed to get rid of.
I never ever thought I'd actually want to get married, being a feminist and all that and having seen my parents' hideous divorce (and my father's horrid second marrige). Shocked myself when, after living with then-dp for two years, I think, I suddenly realised I did want to marry him - to make a public declaration of our love for each other and make it 'official'.
Mind you, we had a very un-traditional wedding - I would have liked a church ceremony because although I'm not uncritical of it I was brought up CofEand love the wedding service. Would have been very happy to promise to cherish dh! But he is allergic to churches (some very dodgy religious relatives - not CofE but enough to put him off religion completely) so we ended up with a civil wedding followed by a pagan ceremony. Which sounds odd for someone who would have been happy with the CofE but it relfected another important part of my admittedly confused belief system (although I'm not a practising pagan, far from it).
What surprised me was how moving I found the registry office service (this was before you were allowed to marry in licensed venues). I had thought of it as merely a piece of paper and the true wedding as the pagan ceremony where we were making some real promises to each other. Yet the words we spoke at the civil ceremony, even though they were legalistic and not beautiful (as Cranmer's English and the King James Bible are), really meant something when I stood up with dp/dh. I cried!
The pagan ceremony was meaningful and significant. I did feel blessed by our guests and by the pagan gods and goddesses - basically nature worship and we got married somewhere where that seems very real (the Peak District). The land, which I love, is very pagan in some way. Can't really explain it.
Out of my parents and immediate family (sisters and their families) we are the only people who are actually married. And we've been together longer than my parents were. Yet I don't think, given time, that my sister's relationships will necessarily be any less important. Certainly their partners are the fathers of their children - what's more important than that?
And I don't see any evidence, from my own friends who admittedly may not be representative, that people go into marriage thinking they'll get divorced if it works out. Most of our friends lived together in their 20s and got married late 20s and are still together nearly a decade later. In fact I only know (well) two people my age who are divorced, strangely.
I had a theory that people of my generation (mid 30s) were the first where divorce among our parents was widespread and therefore we are more likely to stay together as a reaction to what the previous generation did. But listening to other MNers maybe that's not something that applies to the wider world, just my social circle.
As for children of married parents 'doing better' IIRC that was measured by things like academic achievement and job status. But I'm not sure how well the research was designed or how well the results were reported. And it must be biased by the fact that umarried parents are more variable - if you compared unmarried parents in stable long-term relationships with married parents I'm not sure you'd see much difference. Divorce hits people financially and poverty is linked to 'poor' outcomes in that sense, so comparing married with unmarried/divorced parents is bound to show 'poor' outcomes, I guess.