Women have played 5 sets before (Billie Jean King loves to remind people that in the beginning of tennis women also played best out of five, about a century ago), recently between 1984 and 1998 in the finals of the year-ending championships. In 1996, Graf beat Hingis in a thrilling 5-setter..
WTA CEO and Chairman Stacey Allaster is all for it, she has said it more than once to media.
Biggest obstacles are Slam tournament organizers, sexist opinions, and few of the WTA top players (examples: Serena, and Sharapova). Slam tournament organizers are probably the biggest "anti" people on this because of the numerous scheduling problems that would cause during the 4 tournaments. Williams and Sharapova (and several other lower-ranked players) don't want to do it because they are afraid of injuries, or being too tired to play doubles and/or mixed - thus not be able to make that extra income.
Personally, I think it's ridiculous for the fans. I don't know how much a ticket costs for example for the Saturday final of a Slam for women and a Sunday final for men, but if it's the same, I would have zero motivation to buy a women's final ticket where I may risk getting to see 41 minutes match, whereas in the men's final there is at least a larger chance of a turnaround, or one set or two being close. Ticket value is simply not the same. It should be the same for men and women (which is what Allaster would like if it was up to her). Maybe even make men 2 out 3 sets until the quarters and 3 out 5 from there on, and do exactly the same with women; in any outcome that may be, it should be the same for both men and women.