I don't think polygamy is acceptable in some of the cases seen today. The only way I see it being OK is if there's a severe shortage of men (war, etc.) or there's a widow who can't support herself or her children, or one's wife can't have children and the man is, for some reason, totally opposed to adoption. I think that's why polygamy was permitted in the first place, but I think some men who have more than one wife abuse this. Yes, Islam allows it, that doesn't mean you can go and marry 2 or 3 or 4 wives for "fun" (well, it's not all that fun!).
I would be totally uncomfortable with my husband having more than one wife - doesn't matter if I was the first or the last wife. I would hope that my husband would consult me and seek my permission first if he ever wanted to take another wife. Even though he doesn't need my permission, I would never marry a man who seems like he would be that inconsiderate to my feelings. I would want my husbands affection to be going to another woman. And it's impossible to divide your love and affection exactly 50/50.