So these guys have used your photography, from whatever source, and published the magazine without having an agreement in place in advance. This breaks so many copyright issues straight off. Then they send you a contract after the fact and expect you to sign off worldwide rights in all media for all perpetuity!
What do they think, you're going to be so pleased they used your stuff that you'll thank them for the priviledge and hand over all rights?!
I appreciate exposure is a valuable aspect of being published but I think this is pretty insulting to your professionalism and business.
On the up side since they have already gone to press, you are in a strong position for negotiation on fees. I would use it as a way to get paid appropriately and absolutely not sign off any rights other than one time, one issue, print and web, time limited. Any future rights needing to be negotiated seperately and for more fees. As is normal practice.
This type of rights grab is not uncommon, unfortunately. Here in the UK, and maybe internationally, Conde Nast (Vogue, Vanity Fair, GQ, House & Garden, Architectural Digest, et al) tried to force that sort of waiver on it's contributing photographers and met with a lot of resistance from the AoP (Association of Photographers, UK equivalent to the ASMP) who strongly advised against giving in to this sort of pressure, especially as there is no way back from it. I can't say if they are still trying to force this on photographers.
Another thought, who supplied the images, and did they have the licensed (and paid for) right to do so. And from what you said did they have the right to use the images themselves? More fees could be generated there.
Lastly its nice to see that they liked the images enough to use them on the cover. Positive feedback on one's work is always good in this often solitary business. Shame it had to come like this.
Good luck with this and I hope you are able to get a positive outcome from it, as well as getting well paid of course.