I guess in the past marriages stayed together more often because there was stigma against divorcee's, women "having" to work, etc. Then there was the "free love" era, then there was the "me" era. But at the end of the day, it still falls on the couple being honest, responsible and true to themselves. Sometimes things just turn out differently, but people still get so caught up in things that are not as important (and let's face it, the Internet opens up so much more temptation & distraction that its even harder) and the people they should be talking to, getting to know better and confiding in should be the spouse and not some guy/girl you meet on line.
Are marriages better these days? I have lots of friends who are happily married and raising fantastic kids. I have friends who haven't been so lucky and who have fucked up their marriages because they couldn't communicate or just really needed *someone* (whoever) to take care of them. That is what makes me sad though - it always seems like the one you are supposed to love is the one who gets taken the most for granted. Its always so much easier to talk to / confide in other people who you really don't have commitments to as well. At the end of the day being comfortable w/ yourself as a person who can function on his / her own and also the ability to communicate w/ the other person.
Maybe marriage hasn't changed so much as what is "acceptable" in society that distracts from those basic things - responsibility, honesty & communication.