Why should he resign over this? I mean, homosexuality is one thing - it doesn't stop Barney Frank from being elected every 2 years to the House. Having an affair is also one thing - Bill Clinton and a host of others have done it. This just combines the two. What's the big deal?
I think that McGreevey has pulled a "clinton" here.
He was running an extremely corrupt administration, which a great amount of skullduggery that was probably going to be discovered. His affair with his (unqualified) Homeland Security Director is only the tip of the iceberg. There may be some things for which he could be prosecuted.
By resigning the way he did, he shifts the focus from his other crimes to his sexual orientation, and makes it more difficult to go after him for his other activities.