>marriage doesn't stop heterosexual men from cheating
maybe this is true, but you must consider that married people are broadly speaking less likely to sleep around. and there are other benefits to being married. it's not just an arrangement for making babies. the culture should promote pair bonding commitments more aggressively and de-emphasize the role of the state as a substitute for a spouse.
Yes, but this is likely to be a selection effect. If you're going to cheat on your spouse, you would be less likely to get married so that you don't risk an expensive divorce. If you're forced to get married it won't necessarily change your nature.
Social attitudes are driven by politics and legislation. Gay marriage is of course an example of this. Much less popular before it was made legal than after. Same with abortion and a number of other things. Women are never going to vote away their right to initiate financially devastating divorces so the popularity of marriage will likely continue to decline.
